app validate on a path that does not exist reports a missing manifest and exits 1 — the contract says 2

[ZacxDev]

Found by a blind dogfood run against v0.1.90-21-gf56aa72.

Symptom

A path that does not exist is reported as "a project root without a manifest", and exits 1 where the published contract says 2.

$ civitai app validate /nope/does/not/exist ; echo $?
✗ 1 validation error(s) in /nope/does/not/exist:
  - block.manifest.json not found at project root /nope/does/not/exist
Error: validation failed
1

Two problems in one line:

1. Wrong diagnosis. "not found at project root X" states that X is a project root and merely lacks a manifest. The truth is that X is not there at all. A first-time author reads this and starts hunting for a typo in the filename rather than in the path.
2. Wrong exit code. The exit-code contract published from exitCodeDocs draws the split explicitly for a named local path that is not there: it exits 2 (a mistake about the invocation), not 1.

A file that is not a directory is worse — a raw syscall error with a concatenated nonsense path:

$ civitai app validate ./README.md ; echo $?
Error: stat README.md/block.manifest.json: not a directory
1

This is the third copy of an inconsistency already fixed twice

The identical mistake, two commands, after #251:

$ civitai generate --input /nope/x.json --print-input
Error: --input /nope/x.json: no such file — pass a path that exists, or `--input -` to read
the graph from stdin; produce a starting point with `civitai generate "a cat" --print-input`
rc=2

$ civitai app validate /nope
- block.manifest.json not found at project root /nope
rc=1

Same class of error, same user mistake. One names the fix and exits 2; the other is bare and exits 1. #242, #245 and #251 each closed one copy of "a path failure is sorted by the wrong axis"; this is the remaining one, and it sits on the command a new author runs most.

AGENTS.md item 24 is the governing context — the split is by the author's mistake, not by the errno.

Suggested fix

In app validate's path resolution, os.Stat the target before looking for the manifest and branch three ways:

• does not exist → asUsageError (exit 2), message naming the next command (civitai app create, or civitai app validate <dir> if they meant a different directory);
• exists but is not a directory → asUsageError (exit 2), saying so plainly rather than surfacing stat X/block.manifest.json: not a directory;
• is a directory with no manifest → the current message and behaviour, which are correct.

Keep the "directory with no manifest" case as-is: it is a genuine validation finding, not a usage error.

Test coverage this needs

Pin the exit code with errors.Is(err, ErrUsage), never message text — per AGENTS.md item 7, the sentinels carry no visible text, so a message assertion says nothing about the code. Measured on #251: an equivalent one-token mutation passed a green suite because every existing assertion was on wording.

Rows: missing path → 2; a regular file → 2; a real project dir with no manifest → 1 (control, unchanged); a valid project → 0 (control).