feat(daemon): Add --global flag for multi-repo support

Implements bd-121: Global daemon with system-wide socket

Changes:
- Add --global flag to daemon command
- Use ~/.beads/bd.sock when --global is set
- Skip git repo validation for global daemon
- Update daemon discovery to check ~/.beads/ as fallback
- Both Go CLI and Python MCP client check global socket
- Update all tests to pass global parameter

Benefits:
- Single daemon serves all repos on system
- No per-repo daemon management needed
- Better resource usage for users with many repos
- Automatic fallback when local daemon not running

Usage:
  bd daemon --global         # Start global daemon
  bd daemon --status --global # Check global status
  bd daemon --stop --global   # Stop global daemon

Related: bd-73 (multi-repo epic)
Amp-Thread-ID: https://ampcode.com/threads/T-ea606216-b886-4af0-bba8-56d000362d01
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -169,9 +169,24 @@ var rootCmd = &cobra.Command{
 }
 
 // getSocketPath returns the daemon socket path based on the database location
+// If no local socket exists, check for global socket at ~/.beads/bd.sock
 func getSocketPath() string {
-	// Socket lives in same directory as database: .beads/bd.sock
-	return filepath.Join(filepath.Dir(dbPath), "bd.sock")
+	// First check local socket (same directory as database: .beads/bd.sock)
+	localSocket := filepath.Join(filepath.Dir(dbPath), "bd.sock")
+	if _, err := os.Stat(localSocket); err == nil {
+		return localSocket
+	}
+	
+	// Fall back to global socket at ~/.beads/bd.sock
+	if home, err := os.UserHomeDir(); err == nil {
+		globalSocket := filepath.Join(home, ".beads", "bd.sock")
+		if _, err := os.Stat(globalSocket); err == nil {
+			return globalSocket
+		}
+	}
+	
+	// Default to local socket even if it doesn't exist
+	return localSocket
 }
 
 // outputJSON outputs data as pretty-printed JSON