fix(codebuff): timestamp timezone and reconciliation performance

1. Timestamp timezone: Convert UTC session directory timestamp to local
   time before combining with wall-clock timestamps, preventing shifted
   session bounds for non-UTC users.

2. Reconciliation performance: Added Codebuff case to
   providerSourceFreshBeforeFingerprint that computes a composite
   stat-only value across all three files (chat-messages.json,
   run-state.json, chat-meta.json) and checks the skip cache before
   content hashing. This reduces background I/O for unchanged sessions.

Author: scross01

internal/parser/codebuff.go

@@ -411,14 +411,16 @@ func readCodebuffChatMeta(path string) codebuffChatMeta {
 
 // parseCodebuffSessionDate parses the session directory name as an ISO 8601
 // timestamp. The directory name format is "2026-07-16T00-09-00.236Z".
+// The returned time is always in the local timezone so that time-only
+// message timestamps (HH:MM PM) combine correctly with the date.
 func parseCodebuffSessionDate(sessionID string) time.Time {
 	// Try full ISO format with milliseconds and Z suffix.
 	if ts, err := time.Parse("2006-01-02T15-04-05.999Z", sessionID); err == nil {
-		return ts
+		return ts.In(time.Local)
 	}
 	// Try without milliseconds.
 	if ts, err := time.Parse("2006-01-02T15-04-05Z", sessionID); err == nil {
-		return ts
+		return ts.In(time.Local)
 	}
 	// Try with milliseconds, no Z. Interpret as local time since
 	// codebuff records wall-clock timestamps without a UTC offset.

internal/sync/engine.go

@@ -8405,6 +8405,32 @@ func (e *Engine) providerSourceFreshBeforeFingerprint(
 		if e.shouldSkipByPath(path, effectiveInfo) {
 			return mtime, true
 		}
+	case parser.AgentCodebuff:
+		// Codebuff's fingerprint is composite (chat-messages.json
+		// plus run-state.json and chat-meta.json). The stat-only
+		// composite below matches the stored Size/Mtime the fingerprint
+		// stamps, so unchanged sessions skip without reading transcript
+		// bytes, and a sibling-only change still changes the composite
+		// and falls through to the full fingerprint.
+		dir := filepath.Dir(path)
+		size := info.Size()
+		mtime := info.ModTime().UnixNano()
+		for _, name := range []string{"run-state.json", "chat-meta.json"} {
+			companion := filepath.Join(dir, name)
+			if ci, err := os.Stat(companion); err == nil {
+				size += ci.Size()
+				if cm := ci.ModTime().UnixNano(); cm > mtime {
+					mtime = cm
+				}
+			}
+		}
+		effectiveInfo := fakeSnapshotInfo{
+			fSize:  size,
+			fMtime: mtime,
+		}
+		if e.shouldSkipByPath(path, effectiveInfo) {
+			return mtime, true
+		}
 	}
 	return 0, false
 }