Chunk mode + +-N context. doc / span modes return placeholder
errors (filled by subsequent tasks). fmt_canonical_to_markdown
helper introduced now since doc mode (Task 4) consumes it.
Errors are typed StructuredError so classify preserves
chunk_not_found / doc_not_found through the wire layer.
Adds SqliteStore::list_chunk_ids_for_doc so the facade can derive
+-N neighbors without leaking direct rusqlite usage into kebab-app.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>