fix(pipeline,api): write the source page last; say how to clear an unreadable note (review of #211, pass 4)

The source page carries source_hash, which the drift check trusts. Written first,
a batch that failed midway left a hash behind, so re-ingesting the same content
was skipped and the rest of the batch was lost for good. In a wing it is now
written last. An over-cap note in brain_pending says promote/reject cannot act
on it and to delete or move it by hand. Mutation-checked.

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

ingestion/internal/api/promote.go

@@ -105,7 +105,7 @@ func listNotes(root *os.Root, queue string) ([]PendingNote, error) {
 			}
 			out = append(out, PendingNote{
 				Filename: e.Name(), CreatedAt: fi.ModTime().UTC().Format(time.RFC3339), SizeBytes: fi.Size(),
-				Excerpt: "(cannot be read: " + readErr.Error() + ")", Queue: queue,
+				Excerpt: "(cannot be read: " + readErr.Error() + "; promote and reject cannot act on it: delete or move it by hand)", Queue: queue,
 			})
 			continue
 		}