Add contract_gen.py: a pure, deterministic generator that discovers every
StrEnum/dataclass/card defined in contract.py (plus EntryMode,
ConversationItem, and SessionView) via introspection and emits the checked-in
contract_schema.json (JSON Schema draft 2020-12) and
web/.../contract/types.ts. The discriminated ConversationItem union is built
from _Card.__subclasses__(), one oneOf/union branch per card kind.
test_contract.py gains a lockstep test that byte-compares a fresh
generate() call against the checked-in files (guards drift/hand-edits) and a
jsonschema.validate test that proves the generated schema is itself valid
and accepts a real SessionView instance.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>