fix(codebuff): stat-only scheduling fingerprint for incremental sync

Added Codebuff case to discoveredFileEffectiveMtime that computes a
stat-only composite mtime across all three files (chat-messages.json,
run-state.json, chat-meta.json) instead of using the full content-
hashing fingerprint. This reduces background I/O for unchanged sessions
from O(N) file reads to O(1) stat checks during incremental sync cutoff
filtering.

Author: scross01

internal/sync/engine.go

@@ -5309,6 +5309,32 @@ func (e *Engine) discoveredFileEffectiveMtime(
 		}
 		return mtime, nil
 	}
+	// Codebuff is excluded from the provider-Fingerprint path for
+	// cost: its Fingerprint content-hashes chat-messages.json plus
+	// run-state.json and chat-meta.json, so consulting it here would
+	// read every session's full transcript on each incremental sync,
+	// scaling cutoff filtering with the archive instead of the changed
+	// batch. The stat-only composite carries the same cutoff signal —
+	// the max mtime and total size of all three files — so a
+	// companion-only change still looks fresh. Sources that pass the
+	// cutoff go on to the full fingerprint as usual.
+	if file.Agent == parser.AgentCodebuff {
+		info, err := os.Stat(file.Path)
+		if err != nil {
+			return 0, err
+		}
+		mtime := info.ModTime().UnixNano()
+		dir := filepath.Dir(file.Path)
+		for _, name := range []string{"run-state.json", "chat-meta.json"} {
+			companion := filepath.Join(dir, name)
+			if ci, err := os.Stat(companion); err == nil {
+				if ts := ci.ModTime().UnixNano(); ts > mtime {
+					mtime = ts
+				}
+			}
+		}
+		return mtime, nil
+	}
 	// Provider-authoritative sources resolve freshness through the provider
 	// Fingerprint so composite provider-owned source state participates in
 	// incremental-sync cutoff checks.