Add git hooks package and lnk hooks CLI

[polymorcodeus]

Summary

Adds git hook automation for lnk: a post-merge hook in the lnk repo and a post-checkout hook in project repos. Makes ProjectRestore idempotent and introduces collision-safe hook restore paths so hooks can run unattended on every checkout without silently replacing user files.

What's new

• internal/hooks package - installs and uninstalls lnk-managed post-merge and post-checkout scripts, using marker comments to refuse overwriting foreign hooks.
• lnk hooks CLI - install, uninstall, and run commands, with --project targeting a project repo instead of the lnk repo.
• Hook runner - service.RunHook dispatches to collision-safe restore logic and wraps every body in runWithTimeout, so a hung or failing restore never blocks the git operation.
• Idempotent project restore - ProjectRestore now skips symlinks that already resolve to the correct storage target instead of recreating them.
• Collision-safe restore - RestoreHook and ProjectRestoreHook create missing symlinks, replace stale symlinks, and report real-file collisions without backing them up.

Usage

Command	Purpose
lnk hooks install	install the post-merge hook in the lnk repo
lnk hooks install --project	install the post-checkout hook in the current project repo
lnk hooks uninstall	remove lnk's post-merge hook
lnk hooks uninstall --project	remove lnk's post-checkout hook
lnk hooks run <hook-name> [args...]	entry point called by the installed git hook scripts

Behavior changes / guardrails

• Hooks are opt-in; install refuses to overwrite a hook that was not written by lnk.
• post-checkout resolves the project root from the current working directory and refuses to treat the lnk repo (or any clone of it) as a project.
• Hook restore reports real-file collisions to stderr and leaves the files in place; no .lnk-backup is created during hook execution.
• A hung or erroring restore body is capped by a timeout and degraded to a stderr warning.

Testing

• Unit tests cover hooks install/uninstall, RestoreHook, ProjectRestoreHook, RunHook, and project restore idempotency.
• Integration tests exercise real git post-checkout hooks for missing-symlink restore, collision reporting, and idempotent branch switches.
• make check runs fmt, vet, lint, unit, and integration suites; all green.