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

commands/stats.md

@@ -12,6 +12,6 @@ Use the beads MCP `stats` tool to retrieve project metrics and present them clea
 - Recently updated issues
 
 Optionally suggest actions based on the stats:
-- High number of blocked issues? Run `/beads:blocked` to investigate
-- No in-progress work? Run `/beads:ready` to find tasks
-- Many open issues? Consider prioritizing with `/beads:update`
+- High number of blocked issues? Run `/bd-blocked` to investigate
+- No in-progress work? Run `/bd-ready` to find tasks
+- Many open issues? Consider prioritizing with `/bd-update`