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_sync.go

@@ -601,18 +601,6 @@ func performAutoImport(ctx context.Context, store storage.Storage, skipGit bool,
 			return
 		}
 
-		// Update jsonl_content_hash after successful import to prevent repeated imports
-		// Uses repoKey for multi-repo support (bd-ar2.10, bd-ar2.11)
-		hashKey := "jsonl_content_hash"
-		if repoKey != "" {
-			hashKey += ":" + repoKey
-		}
-		if currentHash, err := computeJSONLHash(jsonlPath); err == nil {
-			if err := store.SetMetadata(importCtx, hashKey, currentHash); err != nil {
-				log.log("Warning: failed to update %s after import: %v", hashKey, err)
-			}
-		}
-
 		if skipGit {
 			log.log("Local auto-import complete")
 		} else {