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_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))
+}