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/internal/extract/callsites_test.go

@@ -88,7 +88,6 @@ func TestNoPackageReferencesExtractTextOutsideExtract(t *testing.T) {
 // osOpenAllowed names the functions in guarded files that may call os.Open*
 // on a path that is not a raw/ read.
 var osOpenAllowed = map[string]bool{
-	"internal/watcher/watcher.go:appendWatcherLog": true, // ingest log append, not a raw/ read
 	// Text reads by path; outside this package only TextIn's temp copy reaches
 	// it (TestNoPackageReferencesExtractTextOutsideExtract enforces that).
 	"internal/extract/extract.go:Text": true,