Problem: In git worktrees, .git is a pointer file (not a directory), so cargo's
rerun-if-changed=.git/HEAD never triggers when commits are made. This causes
claw version to report a stale SHA after new commits.
Solution: Add resolve_git_head_path() helper that detects worktree mode:
- If .git is a file: parse gitdir pointer, watch <gitdir>/HEAD
- If .git is a directory: watch .git/HEAD (regular repo)
This ensures build.rs invalidates on each commit, making version output truthful.
Verification: Binary built in worktree now reports correct SHA after commits
(before: stale, after: current HEAD).
Relates to ROADMAP #161 (filed cycle #65, implemented cycle #69).
Diagnostic-strictness family member.
Diff: 21 lines added (resolve_git_head_path + conditional rerun-if-changed).
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