fix: relocate daemon socket for deep paths (GH#1001)

On Unix systems, socket paths are limited to 104 chars (macOS) or 108 chars
(Linux). Deep workspace paths like /Volumes/External Drive/Dropbox/...
would exceed this limit and cause daemon startup failures.

This fix:
- Adds ShortSocketPath() which computes /tmp/beads-{hash}/bd.sock for
  paths that would exceed the limit
- Keeps backward compatibility: short paths still use .beads/bd.sock
- Updates daemon discovery to check both locations
- Uses SHA256 hash of canonical workspace path for unique directories

Closes GH#1001

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/daemon_autostart_unit_test.go

@@ -340,6 +340,11 @@ func TestDaemonAutostart_RestartDaemonForVersionMismatch_Stubbed(t *testing.T) {
 		t.Fatalf("getPIDFilePath: %v", err)
 	}
 	sock := getSocketPath()
+	// Create socket directory if needed (GH#1001 - socket may be in /tmp/beads-{hash}/)
+	sockDir := filepath.Dir(sock)
+	if err := os.MkdirAll(sockDir, 0o750); err != nil {
+		t.Fatalf("MkdirAll sockDir: %v", err)
+	}
 	if err := os.WriteFile(pidFile, []byte("999999\n"), 0o600); err != nil {
 		t.Fatalf("WriteFile pid: %v", err)
 	}