Require a pre-commit fixpoint before AFK commits

[jonathansick]

Summary

• Tighten .claude/skills/afk-implement/SKILL.md Phase 2 so the staging gate is an explicit fixpoint: pre-commit run --all-files must exit 0 and git status --porcelain must be empty in a single run before staging. Drive-by ruff auto-fixes on unrelated files must be staged or reverted, not left dangling.
• Treat "Found N errors (X fixed, Y remaining)" as a failure rather than a success — the agent must hand-fix the unfixable Y, closing the Mode B path that landed PR DM-54689: GitHub push webhook + PushEventProcessor #264's red lint job.
• Add a Phase 3 push-time backstop that re-runs pre-commit run --all-files and git status --porcelain before git push; any non-clean result routes to the stuck path instead of pushing a tree CI will reject.

Test plan

• Re-read the updated afk-implement SKILL.md and confirm the wording leaves no "exits 0 eventually" reading.
• Run an AFK iteration on a small task that touches a directory containing latent ruff violations; confirm the agent either stages the unrelated fix or reverts it before commit.
• After the next Ralph-produced PR, gh pr checks <num> --repo lsst-sqre/docverse should report lint green.
• Run uv run --only-group=lint pre-commit run --all-files on main to confirm the baseline tree is clean (fixpoint semantics depend on it).