Fix P1 deletion tracking bugs (bd-rbxi)

- bd-nqes: Made snapshot capture mandatory with fail-fast
- bd-mn9p: Added semantic JSON comparison (jsonEquals)
- bd-2ifg: Collect deletion errors and fail operation
- bd-8ayj: Atomic file ops with PID-specific temp files
- bd-aewm: Added defer cleanup for .merged temp file

All tests pass.

Amp-Thread-ID: https://ampcode.com/threads/T-5e744954-8a08-4697-960e-5f2a88d50c54
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/sync.go

@@ -150,8 +150,10 @@ Use --merge to merge the sync branch back to main branch.`,
 			}
 
 			// Capture left snapshot (pre-pull state) for 3-way merge
+			// This is mandatory for deletion tracking integrity
 			if err := captureLeftSnapshot(jsonlPath); err != nil {
-				fmt.Fprintf(os.Stderr, "Warning: failed to capture snapshot for deletion tracking: %v\n", err)
+				fmt.Fprintf(os.Stderr, "Error: failed to capture snapshot (required for deletion tracking): %v\n", err)
+				os.Exit(1)
 			}
 		}