fix(api,watcher): name the queue, list parked non-markdown, reserved wings, top-level-only frontmatter key; document the inbox (review of #211)

- brain_promote/brain_reject take an optional queue (inbox | needs-review); a
  name present in both is refused instead of silently acting on the top-level
  note; brain_pending labels every entry with its queue
- parked pdf/txt files are listed (they were invisible); only markdown can be
  promoted, the others are moved or rejected
- promote refuses sources/concepts/entities/wiki as a wing
- frontmatterWing reads only a top-level key inside a properly fenced block
- docs/raw-inbox.md: the contract, the three env vars, the known limits
Each behaviour has a test that fails without it (mutation-checked).

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

ingestion/internal/api/pending_review_test.go

@@ -34,7 +34,7 @@ func TestListPending_ShowsParkedNotesWithTheirProposal(t *testing.T) {
 	assert.NotContains(t, got[0].Excerpt, "proposed_wing", "excerpt is body, not frontmatter")
 
 	assert.Equal(t, "fresh.md", got[1].Filename)
-	assert.Empty(t, got[1].Queue, "a top-level note has no queue label")
+	assert.Equal(t, QueueInbox, got[1].Queue)
 	assert.Empty(t, got[1].ProposedWing)
 }