chore(tooling): migrate .githooks to lefthook.yml

WS-3 session 1a Task 1.

Lefthook installed as root dev-dependency with postinstall = lefthook
install. The two hand-rolled scripts in .githooks/ (post-commit,
pre-push) are dispatched 1:1 from lefthook.yml: each lefthook command
shells out to the existing .githooks/<hook> script. The script bodies
are kept as the source of truth because the bash logic (merge-commit
detection, .claude-sync.conf parsing, non-blocking pre-push warning)
would be lossy to translate into a YAML run: | block.

Active hook path moved from .githooks/ to .git/hooks/ via lefthook
install (core.hooksPath unset, git falls back to its default). The
.githooks/ directory is preserved and now documented as the
implementation invoked by lefthook plus an emergency rollback target
(README added).

Smoke-tested locally: the post-commit hook fires on every commit
(verified by reverted test commit). The pre-push hook fires on every
real push with new commits — manual `lefthook run pre-push` requires
`--force` because lefthook v2 skips when {push_files} is empty (see
lefthook.yml comment).

.githooks/README.md

@@ -0,0 +1,42 @@
+# .githooks/
+
+These scripts contain the actual logic for Crewli's `post-commit` and
+`pre-push` hooks (Claude project-knowledge sync gate, non-blocking
+sync-staleness warning).
+
+## How they're dispatched
+
+Until WS-3 session 1a (2026-04-29) git invoked these scripts directly
+via `git config core.hooksPath .githooks`. From that session onward
+they are dispatched by **lefthook**, configured in `lefthook.yml` at
+the repo root. Lefthook installs its own wrapper scripts into
+`.git/hooks/` and reads `lefthook.yml` to decide what to run; for
+this repo the wrapper invokes the same `bash .githooks/<hook>` calls
+that git used to make.
+
+The migration is 1:1 by design — the hook implementations live here
+because the bash logic (merge-commit handling, conf parsing,
+non-blocking warning) is intricate enough that re-translating it
+into a YAML `run: |` block would be lossy.
+
+## Re-installing lefthook
+
+```bash
+pnpm install   # postinstall script runs `lefthook install`
+```
+
+## Emergency rollback
+
+If lefthook is broken and you need git to invoke these scripts
+directly again:
+
+```bash
+git config core.hooksPath .githooks
+```
+
+To return to lefthook:
+
+```bash
+git config --unset --local core.hooksPath
+pnpm exec lefthook install
+```