Fix bd-v0y: Remove mtime fast-path in hasJSONLChanged

Git doesn't preserve mtime on checkout, causing false negatives where
hasJSONLChanged() incorrectly returns false after git pull updates JSONL.
This caused bd sync to overwrite pulled JSONL instead of importing it,
resurrecting deleted issues.

Solution: Always compute content hash for comparison (Option 1).
Performance impact is minimal (~10-50ms for sync operations).

Changes:
- cmd/bd/integrity.go: Remove mtime fast-path, always compute hash
- cmd/bd/sync.go: Remove mtime storage after import
- cmd/bd/import.go: Remove mtime storage after import
- cmd/bd/daemon_sync.go: Remove mtime storage and update comments
- cmd/bd/daemon_sync_test.go: Remove mtime assertions from tests

All tests pass. Existing test 'mtime changed but content same - git
operation scenario' verifies the fix works correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/import.go

@@ -337,14 +337,7 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 					// Non-fatal warning (see above comment about graceful degradation)
 					debug.Logf("Warning: failed to update last_import_time: %v", err)
 				}
-				// Store mtime for fast-path optimization in hasJSONLChanged (bd-3bg)
-				if jsonlInfo, statErr := os.Stat(input); statErr == nil {
-					mtimeStr := fmt.Sprintf("%d", jsonlInfo.ModTime().Unix())
-					if err := store.SetMetadata(ctx, "last_import_mtime", mtimeStr); err != nil {
-						// Non-fatal warning (see above comment about graceful degradation)
-						debug.Logf("Warning: failed to update last_import_mtime: %v", err)
-					}
-				}
+				// Note: mtime tracking removed in bd-v0y fix (git doesn't preserve mtime)
 			} else {
 				debug.Logf("Warning: failed to read JSONL for hash update: %v", err)
 			}