fix: read audit receipts without cat

[brickfrog]

Eliminates per-tick cat receipt subprocesses by threading injected path_exists/read_file through merge and advisory validation while preserving git tree resolution via capture. Adds missing/valid/no-cat coverage and updates existing receipt fakes. Verification: moon test --target native (2283 passed); moon run src/bin/choir_lint --target native passed. No broader capture_command_stdout hygiene sweep was performed; it remains out of scope.

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon test --target native
  • exit: 0
  • head: 2d9f74a
  • output tail:

----- BEG4
IN MOON TEST RESULT -----
{"type":"start","file":"exec_test.mbt","index":9}
----- END MOON TEST RESULT -----
Total tests: 2283, passed: 2283, failed: 0.

• moon run src/bin/choir_lint --target native
  • exit: 0
  • head: 2d9f74a

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).