fix(watcher): keep a note that changed while it was being ingested

The archive holds the content read at the start; if Syncthing replaced the
note during the (minutes-long) LLM pass, removing the original would drop the
newer content. Compare size+mtime before removing; leave it for the next poll.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ingestion/internal/watcher/watcher.go

@@ -156,6 +156,10 @@ func processFile(ctx context.Context, cfg Config, root *os.Root, rawDir, path, d
 		return fmt.Errorf("relative path: %w", err)
 	}
 
+	before, err := root.Lstat(rel)
+	if err != nil {
+		return fmt.Errorf("stat before ingest: %w", err)
+	}
 	content, err := extract.TextIn(root, rel)
 	if err != nil {
 		return fmt.Errorf("extract text: %w", err)
@@ -172,6 +176,10 @@ func processFile(ctx context.Context, cfg Config, root *os.Root, rawDir, path, d
 		if cpErr := copyFromRoot(root, rel, dest); cpErr != nil {
 			return fmt.Errorf("copy to failed: %w", cpErr)
 		}
+		if changedSince(root, rel, before) {
+			slog.Warn("watcher: note changed during ingest; leaving it in raw/ for the next poll", "file", filename)
+			return nil
+		}
 		retire(cfg, root, rel, path, ".failed", runErr.Error())
 
 		slog.Warn("watcher: file failed", "file", filename, "error", runErr)
@@ -198,12 +206,27 @@ func processFile(ctx context.Context, cfg Config, root *os.Root, rawDir, path, d
 	if err := copyFromRoot(root, rel, dest); err != nil {
 		return fmt.Errorf("copy to processed: %w", err)
 	}
+	if changedSince(root, rel, before) {
+		slog.Warn("watcher: note changed during ingest; leaving it in raw/ for the next poll", "file", filename)
+		return nil
+	}
 	retire(cfg, root, rel, path, ".processed", date)
 
 	slog.Info("watcher: file processed", "file", filename, "source", source)
 	return nil
 }
 
+// changedSince reports whether the note at rel differs (size or mtime) from
+// before. A note replaced mid-ingest (Syncthing) has newer content than the
+// archive copy, so it must not be removed.
+func changedSince(root *os.Root, rel string, before os.FileInfo) bool {
+	now, err := root.Lstat(rel)
+	if err != nil {
+		return true
+	}
+	return now.Size() != before.Size() || !now.ModTime().Equal(before.ModTime())
+}
+
 // retire removes an archived original from raw/ (inbox zero). If the removal
 // fails it falls back to a marker file so the watcher still skips the file.
 func retire(cfg Config, root *os.Root, rel, path, markerSuffix, markerBody string) {

ingestion/internal/watcher/watcher_test.go

@@ -444,3 +444,27 @@ func TestProcessDir_KeepsTheOriginalWhenTheArchiveCopyFails(t *testing.T) {
 	_, err := os.Stat(note)
 	assert.NoError(t, err, "original stays in raw/ when it could not be archived")
 }
+
+// If the note is replaced (e.g. by Syncthing) while it is being ingested, the
+// archive holds the OLD content; removing the original would lose the new one.
+func TestProcessDir_KeepsANoteThatChangedWhileItWasBeingIngested(t *testing.T) {
+	brainDir := setupBrainDir(t)
+	note := filepath.Join(brainDir, "raw", "edited.md")
+	require.NoError(t, os.WriteFile(note, []byte("old"), 0o644))
+
+	cfg := Config{
+		BrainDir: brainDir,
+		Pipeline: pipeline.Config{
+			Complete: func(ctx context.Context, s, u string) (string, error) {
+				require.NoError(t, os.WriteFile(note, []byte("newer and longer content"), 0o644))
+				return successComplete(pipeline.RawPage{Title: "Edited", Type: "source", Subtype: "article", Content: "x\n"})(ctx, s, u)
+			},
+			Schema: "# Schema",
+		},
+	}
+	processDir(context.Background(), cfg, "2026-10-19")
+
+	got, err := os.ReadFile(note)
+	require.NoError(t, err, "a note that changed during ingest must stay in raw/")
+	assert.Equal(t, "newer and longer content", string(got))
+}