Closes #173

Summary

• Derive MCP marker-eligible fields from the shell and content field sets.
• Add marked and unmarked cmd and shell_command regression coverage.
• Assert that a marker in an MCP path cannot override a dangerous command.

Testing

• npm --prefix src/hooks run check
• npm --prefix src/hooks exec vitest run tests/dangerous-actions.test.ts (201 passed)
• python3 scripts/pre_commit.py with a writable temporary npm cache (35 files, 1,446 hook tests passed)

Assumptions

• MCP_PATH_FIELDS remains intentionally excluded from marker eligibility, as specified in the approved plan.
• The extracted runner has no root Python virtual environment, so repository Python checks were skipped by the existing validation script; hook validation completed.