feat: ultraworkers#156 — error classification for text-mode output (Phase 2 of ultraworkers#77)

## Problem

ultraworkers#77 Phase 1 added machine-readable error `kind` discriminants to JSON error
payloads. Text-mode (stderr) errors still emit prose-only output with no
structured classification.

Observability tools (log aggregators, CI error parsers) parsing stderr can't
distinguish error classes without regex-scraping the prose.

## Fix

Added `[error-kind: <class>]` prefix line to all text-mode error output.
The prefix appears before the error prose, making it immediately parseable by
line-based log tools without any substring matching.

**Examples:**

## Impact

- Stderr observers (log aggregators, CI systems) can now parse error class
  from the first line without regex or substring scraping
- Same classifier function used for JSON (ultraworkers#77 P1) and text modes
- Text-mode output remains human-readable (error prose unchanged)
- Prefix format follows syslog/structured-logging conventions

## Tests

All 179 rusty-claude-cli tests pass. Verified on 3 different error classes.

Closes ROADMAP ultraworkers#156.

Author: code-yeongyu

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

@@ -223,14 +223,21 @@ fn main() {
                     "hint": hint,
                 })
             );
-        } else if message.contains("`claw --help`") {
-            eprintln!("error: {message}");
         } else {
-            eprintln!(
-                "error: {message}
+            // #156: Add machine-readable error kind to text output so stderr observers
+            // don't need to regex-scrape the prose.
+            let kind = classify_error_kind(&message);
+            if message.contains("`claw --help`") {
+                eprintln!("[error-kind: {kind}]
+error: {message}");
+            } else {
+                eprintln!(
+                    "[error-kind: {kind}]
+error: {message}
 
 Run `claw --help` for usage."
-            );
+                );
+            }
         }
         std::process::exit(1);
     }