Adds llm/_canonical.py with canonical_message_json() and
message_content_hash(). The serialization is the hash contract for the
incoming DAG-shaped message store; snapshot tests in
tests/test_canonical.py pin the wire format.
The include_provider_metadata parameter is threaded through now so a
future semantic_hash column can be added without refactoring
(see plans/dag-provider-metadata-hashing.md).
Also commits the design docs:
- plans/dag-schema.md — full DAG storage design
- plans/dag-provider-metadata-hashing.md — Option 1 now, Option 2 later
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Simon Willison