Fix bd-73: Add git worktree detection and warnings

- Implement robust worktree detection using git-dir vs git-common-dir comparison
- Add prominent warning when daemon mode is active in a worktree
- Warn in 3 places: initial connection, auto-start, and daemon start command
- Show shared database path and clarify BEADS_AUTO_START_DAEMON behavior
- Document limitations and solutions in README.md and AGENTS.md
- Add comprehensive tests for detection and path truncation

Fixes #55

Amp-Thread-ID: https://ampcode.com/threads/T-254eb9e3-1a42-42d7-afdf-b7ca2d2dcb8b
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/daemon.go

@@ -117,6 +117,19 @@ Use --health to check daemon health and metrics.`,
 			os.Exit(1)
 		}
 
+		// Warn if starting daemon in a git worktree
+		if !global {
+			// Ensure dbPath is set for warning
+			if dbPath == "" {
+				if foundDB := beads.FindDatabasePath(); foundDB != "" {
+					dbPath = foundDB
+				}
+			}
+			if dbPath != "" {
+				warnWorktreeDaemon(dbPath)
+			}
+		}
+
 		// Start daemon
 		scope := "local"
 		if global {