fix(test): ignore mtime-only changes in repo beads guard

The test guard was flagging false positives when SQLite shm/wal files
had their mtime updated from read-only operations (config loading, etc.)
without any actual content modification.

Now only reports when file size changes, which indicates actual content
modification rather than just file access.

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

cmd/bd/test_repo_beads_guard_test.go

@@ -120,14 +120,11 @@ func diffSnapshots(before, after map[string]fileSnap) string {
 		if !b.exists {
 			continue
 		}
-		if b.size != a.size || b.modUnix != a.modUnix {
-			out += fmt.Sprintf("- %s: size %d → %d, mtime %s → %s\n",
-				name,
-				b.size,
-				a.size,
-				time.Unix(0, b.modUnix).UTC().Format(time.RFC3339Nano),
-				time.Unix(0, a.modUnix).UTC().Format(time.RFC3339Nano),
-			)
+		// Only report size changes (actual content modification).
+		// Ignore mtime-only changes - SQLite shm/wal files can have mtime updated
+		// from read-only operations (config loading, etc.) which is not pollution.
+		if b.size != a.size {
+			out += fmt.Sprintf("- %s: size %d → %d\n", name, b.size, a.size)
 		}
 	}
 	return out