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

docs/CONFIG.md

@@ -34,7 +34,6 @@ Tool-level settings you can configure:
 | `no-daemon` | `--no-daemon` | `BD_NO_DAEMON` | `false` | Force direct mode, bypass daemon |
 | `no-auto-flush` | `--no-auto-flush` | `BD_NO_AUTO_FLUSH` | `false` | Disable auto JSONL export |
 | `no-auto-import` | `--no-auto-import` | `BD_NO_AUTO_IMPORT` | `false` | Disable auto JSONL import |
-| `no-install-hooks` | `--skip-hooks` | `BD_NO_INSTALL_HOOKS` | `false` | Skip git hook installation during `bd init` |
 | `db` | `--db` | `BD_DB` | (auto-discover) | Database path |
 | `actor` | `--actor` | `BD_ACTOR` | `$USER` | Actor name for audit trail |
 | `flush-debounce` | - | `BEADS_FLUSH_DEBOUNCE` | `5s` | Debounce time for auto-flush |
@@ -60,10 +59,6 @@ flush-debounce: 10s
 # Auto-start daemon (default true)
 auto-start-daemon: true
 
-# Disable git hook installation during bd init
-# (useful if you manage hooks with pre-commit, husky, etc.)
-no-install-hooks: true
-
 # Daemon log rotation settings
 daemon-log-max-size: 50      # MB per file (default 50)
 daemon-log-max-backups: 7    # Number of old logs to keep (default 7)