Keep the commit helper on live platform source

[hamzamerzic]

Summary

• replace the image-baked command at app startup with a tiny launcher to the helper in the live platform checkout
• keep one intuitive pm-commit command while later helper updates activate automatically, without agent preflight calls
• preserve the usable baked fallback if a damaged live checkout is missing the helper
• cover stale-copy replacement, argument forwarding, future live updates, and missing-source fallback

This is a follow-up to #335, which fixed the helper's exact-path behavior. It closes the remaining activation gap where the command on PATH could still be an older baked copy.

Testing

• python3 -m py_compile backend/app/main.py backend/tests/test_pm_commit.py
• bash -n backend/scripts/entrypoint.sh
• scripts/wt-pytest.sh tests/test_pm_commit.py tests/test_test_entrypoint.py tests/test_routes_init_resilience.py -q (14 passed)
• scripts/test.sh --fast (60 passed)
• complete host backend sweep on the preceding reviewed revision, excluding the host-incompatible no-clone route fixture (3,397 passed, 4 skipped, 1 deselected); focused and fast gates rerun after the final fallback guard
• containerized full gate unavailable in this runtime; required CI on submission