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

cmd/bd/daemon_autostart.go

@@ -183,18 +183,7 @@ func acquireStartLock(lockPath, socketPath string) bool {
 	// nolint:gosec // G304: lockPath is derived from secure beads directory
 	lockFile, err := os.OpenFile(lockPath, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0600)
 	if err != nil {
-		// Lock file exists - check if it's from a dead process (stale) or alive daemon
-		lockPID, pidErr := readPIDFromFile(lockPath)
-		if pidErr != nil || !isPIDAlive(lockPID) {
-			// Stale lock from crashed process - clean up immediately (avoids 5s wait)
-			debugLog("startlock is stale (PID %d dead or unreadable), cleaning up", lockPID)
-			_ = os.Remove(lockPath)
-			// Retry lock acquisition after cleanup
-			return acquireStartLock(lockPath, socketPath)
-		}
-
-		// PID is alive - daemon is legitimately starting, wait for socket to be ready
-		debugLog("another process (PID %d) is starting daemon, waiting for readiness", lockPID)
+		debugLog("another process is starting daemon, waiting for readiness")
 		if waitForSocketReadiness(socketPath, 5*time.Second) {
 			return true
 		}