Note the lefthook pre-push manual-run caveat (needs --all-files)

[sksizer]

Summary

• Documents the lefthook pre-push manual-run caveat so a hand-run gate check isn't misread as passing.
• lefthook.yml: adds a "Manual run caveat" note to the pre-push: comment block — bunx lefthook run pre-push reports "no matching push files" and skips its gates on a branch with no upstream / no unpushed commits, so a by-hand check needs --all-files (or a real git push).
• README.md: appends an equivalent, consistently-worded caveat sentence to the Git hooks bullet. Comment/docs-only — no hook run: command or gate behavior changes.

Task

docs/planning/tasks/T-TH8U-document-lefthook-prepush-run-caveat.md

Acceptance criteria

• AC-1: lefthook.yml notes that bunx lefthook run pre-push skips its gates on a branch with no unpushed commits and that --all-files (or a real git push) is required by hand. (auto — verified in git diff origin/main -- lefthook.yml)
• AC-2: The README Git hooks section carries an equivalent caveat sentence. (auto — verified in git diff origin/main -- README.md)
• AC-3: No hook run: command or gate behavior changes — diff limited to a comment in lefthook.yml and prose in README.md. (auto — two-dot diff is +12/-1, run: line unchanged; baseline-gated sdlc quality run reported OK 6/6, no new drift)

Test plan

• nothing requires manual verification — see post-mortem in task file

Follow-up tasks spawned

No new tasks spawned. One friction bullet (Step 7 quality run --diff-against-baseline resolving --baseline-dir to the worktree instead of the superproject) was linked to the existing upstream task T-F1WJ-quality-run-resolves-superproject-baseline (sksizer/dev) rather than duplicated.