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/init.md

@@ -12,7 +12,7 @@ Use the beads MCP `init` tool with the prefix parameter (if provided) to set up
 After initialization:
 1. Show the database location
 2. Show the issue prefix that will be used
-3. Explain the basic workflow (or suggest running `/beads:workflow`)
-4. Suggest creating the first issue with `/beads:create`
+3. Explain the basic workflow (or suggest running `/bd-workflow`)
+4. Suggest creating the first issue with `/bd-create`
 
 If beads is already initialized, inform the user and show project stats using the `stats` tool.