feat(daemon): add BD_SOCKET env var for test isolation (#914)

Add BD_SOCKET environment variable support to override daemon socket path,
enabling parallel test isolation via t.TempDir() + t.Setenv().

Changes:
- getSocketPath() checks BD_SOCKET first, falls back to dbPath-derived path
- getSocketPathForPID() checks BD_SOCKET first (for consistency)
- Add daemon_socket_test.go with isolation pattern examples

This is a minimal tracer bullet to validate the approach before
expanding to full test isolation infrastructure.

Backward compatible: default behavior unchanged without env var set.

cmd/bd/daemon_autostart.go

@@ -455,9 +455,14 @@ func recordDaemonStartFailure() {
 	// No cap needed - backoff is capped at 120s in canRetryDaemonStart
 }
 
-// getSocketPath returns the daemon socket path based on the database location
+// getSocketPath returns the daemon socket path based on the database location.
+// If BD_SOCKET env var is set, uses that value instead (enables test isolation).
 // Returns local socket path (.beads/bd.sock relative to database)
 func getSocketPath() string {
+	// Check environment variable first (enables test isolation)
+	if socketPath := os.Getenv("BD_SOCKET"); socketPath != "" {
+		return socketPath
+	}
 	return filepath.Join(filepath.Dir(dbPath), "bd.sock")
 }