feat: add Git worktree compatibility (PR #478)

Adds comprehensive Git worktree support for beads issue tracking:

Core changes:
- New internal/git/gitdir.go package for worktree detection
- GetGitDir() returns proper .git location (main repo, not worktree)
- Updated all hooks to use git.GetGitDir() instead of local helper
- BeadsDir() now prioritizes main repository's .beads directory

Features:
- Hooks auto-install in main repo when run from worktree
- Shared .beads directory across all worktrees
- Config option no-install-hooks to disable auto-install
- New bd worktree subcommand for diagnostics

Documentation:
- New docs/WORKTREES.md with setup instructions
- Updated CHANGELOG.md and AGENT_INSTRUCTIONS.md

Testing:
- Updated tests to use exported git.GetGitDir()
- Added worktree detection tests

Co-authored-by: Claude <noreply@anthropic.com>
Closes: #478

AGENT_INSTRUCTIONS.md

@@ -77,7 +77,7 @@ The 30-second debounce provides a **transaction window** for batch operations -
 
 **Protected branches**: Use `bd init --branch beads-metadata` to commit to separate branch. See [docs/PROTECTED_BRANCHES.md](docs/PROTECTED_BRANCHES.md).
 
-**Git worktrees**: Daemon mode NOT supported. Use `bd --no-daemon` for all commands. See [docs/GIT_INTEGRATION.md](docs/GIT_INTEGRATION.md).
+**Git worktrees**: Enhanced support with shared database architecture. Use `bd --no-daemon` if daemon warnings appear. See [docs/GIT_INTEGRATION.md](docs/GIT_INTEGRATION.md).
 
 **Merge conflicts**: Rare with hash IDs. If conflicts occur, use `git checkout --theirs/.beads/beads.jsonl` and `bd import`. See [docs/GIT_INTEGRATION.md](docs/GIT_INTEGRATION.md).