fix: route existing PR body updates through file_pr

[brickfrog]

Fixes choir-q6z0. Adds leaf prompt guidance that PR lifecycle writes are restricted to Choir tools/human paths across gh and GitHub App/connector surfaces, and makes file_pr update an existing open PR body via injected gh pr edit --body while logging edit failures non-fatally.\n\nVerification: moon test --target native && moon run src/bin/choir_lint --target native (pass: 2264 tests); moon fmt --check (pass).

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon test --target native
  • exit: 0
  • head: 49c2241
  • output tail:

----- BEG4
IN MOON TEST RESULT -----
{"type":"start","file":"exec_test.mbt","index":9}
----- END MOON TEST RESULT -----
Total tests: 2264, passed: 2264, failed: 0.

• moon run src/bin/choir_lint --target native
  • exit: 0
  • head: 49c2241

[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).