fix(watcher): refusals end in a terminal state; pin every converted writer (#158)

Review round 1 on PR #195:
- A refused marker or archive write left no marker, so the note was
  reprocessed every interval (archive copies piling up; an LLM call per
  round on the failure path). The marker check now uses Lstat (any entry
  at the marker path counts, so a dangling marker link skips the note
  instead of looping), and a note whose archive copy or marker write is
  refused is remembered (path, size, mtime) and not retried until it
  changes.
- cleanRel no longer refuses a backslash: it is a legal filename byte on
  Linux, not a traversal, and a raw note named with one was never
  archived or marked.
- Tests now reach every converted writer: the log.md append (real
  raw/failed so the log is reached, symlinked log.md), a dangling marker
  link, the refused-retry loop with a positive control, a backslash name,
  a buildAndWrite swap race, and the claude-session archive with in-brain
  and outside symlinked archive dirs. The stale lint allowlist entry for
  appendWatcherLog is removed.

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

ingestion/cmd/server/main_test.go

@@ -76,3 +76,27 @@ func TestAlertingWarnings(t *testing.T) {
 		})
 	}
 }
+
+// hyperguild#158: the claude-session archive was written with os.MkdirAll +
+// os.WriteFile by path, so a symlinked archive directory redirected it.
+func TestClaudeSinkDoesNotWriteThroughASymlinkedArchiveDir(t *testing.T) {
+	for name, mk := range map[string]func(t *testing.T, dir string) (link, target string){
+		"outside": func(t *testing.T, _ string) (string, string) { o := t.TempDir(); return o, o },
+		"in-brain": func(t *testing.T, dir string) (string, string) {
+			require.NoError(t, os.MkdirAll(filepath.Join(dir, "raw"), 0o755))
+			return filepath.Join("..", "raw"), filepath.Join(dir, "raw")
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			dir := t.TempDir()
+			link, target := mk(t, dir)
+			require.NoError(t, os.Symlink(link, filepath.Join(dir, "archive")))
+			sink := &claudeSink{brainDir: dir, logger: slog.New(slog.NewTextHandler(os.Stderr, nil))}
+			err := sink.Ingest(context.Background(), claudewatcher.Batch{Host: "koala", SessionID: "abc",
+				Turns: []claudewatcher.Turn{{Type: "assistant", Content: "x"}}})
+			assert.Error(t, err)
+			ents, _ := os.ReadDir(target)
+			assert.Empty(t, ents, "the archive was written at the link's target")
+		})
+	}
+}