feat: ultraworkers#147 — reject empty / whitespace-only prompts at CLI fallthrough

## Problem

The `"prompt"` subcommand arm enforced `if prompt.trim().is_empty()`
and returned a specific error. The fallthrough `other` arm in the same
match block — which routes any unrecognized first positional arg to
`CliAction::Prompt` — had no such guard. Result:

$ claw ""
error: missing Anthropic credentials; export ANTHROPIC_AUTH_TOKEN ...

$ claw "   "
error: missing Anthropic credentials; ...

$ claw "" ""
error: missing Anthropic credentials; ...

$ claw --output-format json ""
{"error":"missing Anthropic credentials; ...","type":"error"}

An empty prompt should never reach the credentials check. Worse: with
valid credentials, the literal empty string gets sent to Claude as a
user prompt, either burning tokens for nothing or triggering a model-
side refusal. Same prompt-misdelivery family as ultraworkers#145.

## Root cause

In `parse_subcommand()`, the final `other =>` arm in the top-level
match only guards against typos (ultraworkers#108 guard via `looks_like_subcommand_typo`)
and then unconditionally builds `CliAction::Prompt { prompt: rest.join(" ") }`.
An empty/whitespace-only join passes through.

## Changes

### rust/crates/rusty-claude-cli/src/main.rs

Added the same `if joined.trim().is_empty()` guard already used in the
`"prompt"` arm to the fallthrough path. Error message distinguishes it
from the `prompt` subcommand path:

  empty prompt: provide a subcommand (run `claw --help`) or a
  non-empty prompt string

Runs AFTER the typo guard (so `claw sttaus` still suggests `status`)
and BEFORE CliAction::Prompt construction (so no network call ever
happens for empty inputs).

### Regression tests

Added 4 assertions in the existing parse_args test:
- parse_args([""]) → Err("empty prompt: ...")
- parse_args(["   "]) → Err("empty prompt: ...")
- parse_args(["", ""]) → Err("empty prompt: ...")
- parse_args(["sttaus"]) → Err("unknown subcommand: ...") [verifies ultraworkers#108 typo guard still takes precedence]

### ROADMAP.md

Added Pinpoint ultraworkers#147 documenting the gap, verification, root cause,
fix shape, and acceptance. Joins the prompt-misdelivery cluster
alongside ultraworkers#145.

## Live verification

$ claw ""
error: empty prompt: provide a subcommand (run `claw --help`) or a non-empty prompt string

$ claw "   "
error: empty prompt: provide a subcommand (run `claw --help`) or a non-empty prompt string

$ claw --output-format json ""
{"error":"empty prompt: provide a subcommand ...","type":"error"}

$ claw prompt ""   # unchanged: subcommand-specific error preserved
error: prompt subcommand requires a prompt string

$ claw hello        # unchanged: typo guard still fires
error: unknown subcommand: hello.
  Did you mean     help

$ claw "real prompt here"   # unchanged: real prompts still reach API
error: api returned 401 Unauthorized (with dummy key, as expected)

All empty/whitespace-only paths exit 1. No network call. No misleading
credentials error.

## Tests

- rusty-claude-cli bin: 177 tests pass (4 new assertions)
- Full workspace green except pre-existing resume_latest flake (unrelated)

Closes ROADMAP ultraworkers#147.

Author: code-yeongyu

ROADMAP.md

@@ -5619,3 +5619,49 @@ Meanwhile `agents`, `mcp`, `skills`, `status`, `doctor`, `sandbox`, `plugins` (a
 **Not applying to.** `hooks` (session-state-modifying, explicitly flagged "unsupported resumed slash command" in main.rs), `usage`, `context`, `tasks`, `theme`, `voice`, `rename`, `copy`, `color`, `effort`, `branch`, `rewind`, `ide`, `tag`, `output-style`, `add-dir` — all session-mutating or interactive-only.
 
 **Source.** Jobdori dogfood 2026-04-21 20:03 KST on main HEAD `7d63699` in response to Clawhip nudge. Joins **surface asymmetry** cluster (#145 sibling). Session tally: ROADMAP #146.
+
+## Pinpoint #147. `claw ""` / `claw "   "` silently fall through to prompt-execution path; empty-prompt guard is subcommand-only
+
+**Gap.** The explicit `claw prompt ""` path rejects empty/whitespace-only prompts with a clear error (`prompt subcommand requires a prompt string`, exit 1, no network call). The implicit fallthrough path — where any unrecognized first positional arg is treated as a prompt — has no such guard. Result: `claw ""`, `claw "   "`, and `claw "" ""` all get routed to the Anthropic call with an empty prompt string, which surfaces the misleading `missing Anthropic credentials` error.
+
+**Verified on main HEAD `f877aca` (2026-04-21 20:32 KST):**
+
+```
+$ claw prompt ""
+error: prompt subcommand requires a prompt string
+
+$ claw ""
+error: missing Anthropic credentials; export ANTHROPIC_AUTH_TOKEN or ANTHROPIC_API_KEY ...
+
+$ claw "   "
+error: missing Anthropic credentials; ...
+
+$ claw "" ""
+error: missing Anthropic credentials; ...
+
+$ claw --output-format json ""
+{"error":"missing Anthropic credentials; ...","type":"error"}
+```
+
+With valid credentials, the empty string would be sent to Claude as a user prompt — burning tokens for nothing, or getting a model-side refusal for empty input.
+
+**Why this is a clawability gap.**
+1. **Inconsistent guard**: the `"prompt"` subcommand arm enforces `if prompt.trim().is_empty() { Err(...) }`, but the fallthrough `other` arm in the same match block does not. Same contract should apply to both paths.
+2. **Prompt misdelivery (Clawhip category)**: same root pattern as #145 (wrong thing gets treated as a prompt). Different manifestation — here it's an empty string, not a typo'd subcommand.
+3. **Misleading error surface**: user sees `missing Anthropic credentials` for a request that should never have reached the API layer at all.
+4. **Clawhip risk**: a misconfigured orchestrator passing `""` or `"   "` as a positional arg ends up paying API costs for empty prompts instead of getting fast feedback.
+
+**Fix shape (~5 lines).** In `parse_subcommand()`'s fallthrough `other` arm, add the same trim-based empty check already used in the `"prompt"` arm, with a message that distinguishes it from the `prompt` subcommand path (e.g. `"empty prompt: provide a command or non-empty prompt text"`). Happens before `looks_like_subcommand_typo` since typos aren't empty.
+
+**Acceptance.**
+- `claw ""` exits 1 with a clear "empty prompt" error, no credential check.
+- `claw "   "` exits 1 with the same error.
+- `claw "" ""` exits 1 with the same error.
+- `claw --output-format json ""` emits the error in typed envelope, exit 1.
+- `claw hello` still reaches the typo guard (#108), not the empty guard.
+- `claw prompt ""` still emits its own specific error.
+- Regression test: `parse_args([""])` → Err, `parse_args(["   "])` → Err.
+
+**Blocker.** None. 5-line change in `parse_subcommand()`.
+
+**Source.** Jobdori dogfood 2026-04-21 20:32 KST on main HEAD `f877aca` in response to Clawhip nudge. Joins **prompt misdelivery** cluster (#145 sibling). Session tally: ROADMAP #147.

rust/crates/rusty-claude-cli/src/main.rs

@@ -824,8 +824,21 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                     return Err(message);
                 }
             }
+            // #147: guard empty/whitespace-only prompts at the fallthrough
+            // path the same way `"prompt"` arm above does. Without this,
+            // `claw ""`, `claw "   "`, and `claw "" ""` silently route to
+            // the Anthropic call and surface a misleading
+            // `missing Anthropic credentials` error (or burn API tokens on
+            // an empty prompt when credentials are present).
+            let joined = rest.join(" ");
+            if joined.trim().is_empty() {
+                return Err(
+                    "empty prompt: provide a subcommand (run `claw --help`) or a non-empty prompt string"
+                        .to_string(),
+                );
+            }
             Ok(CliAction::Prompt {
-                prompt: rest.join(" "),
+                prompt: joined,
                 model,
                 output_format,
                 allowed_tools,
@@ -9743,6 +9756,36 @@ mod tests {
                 output_format: CliOutputFormat::Json,
             }
         );
+        // #147: empty / whitespace-only positional args must be rejected
+        // with a specific error instead of falling through to the prompt
+        // path (where they surface a misleading "missing Anthropic
+        // credentials" error or burn API tokens on an empty prompt).
+        let empty_err = parse_args(&["".to_string()])
+            .expect_err("empty positional arg should be rejected");
+        assert!(
+            empty_err.starts_with("empty prompt:"),
+            "empty-arg error should be specific, got: {empty_err}"
+        );
+        let whitespace_err = parse_args(&["   ".to_string()])
+            .expect_err("whitespace-only positional arg should be rejected");
+        assert!(
+            whitespace_err.starts_with("empty prompt:"),
+            "whitespace-only error should be specific, got: {whitespace_err}"
+        );
+        let multi_empty_err = parse_args(&["".to_string(), "".to_string()])
+            .expect_err("multiple empty positional args should be rejected");
+        assert!(
+            multi_empty_err.starts_with("empty prompt:"),
+            "multi-empty error should be specific, got: {multi_empty_err}"
+        );
+        // Typo guard from #108 must still take precedence for non-empty
+        // single-word non-prompt-looking inputs.
+        let typo_err = parse_args(&["sttaus".to_string()])
+            .expect_err("typo'd subcommand should be caught by #108 guard");
+        assert!(
+            typo_err.starts_with("unknown subcommand:"),
+            "typo guard should fire for 'sttaus', got: {typo_err}"
+        );
     }
 
     #[test]