fix(opencode): read session fallback model from the real schema

[EuanTop]

Summary

• query the real session.model column for SQLite token fallback
• decode the JSON model object into providerID/id
• invalidate stale OpenCode and KiloCode zero-result caches
• update the OpenCode fixture to match the real schema

Context

#394 introduced the session-level fallback using model_id. #404 later introduced the shared OpenCode/KiloCode SQLite parser, and OpenCode was subsequently migrated onto it. Both current schemas store the session model as JSON in model.

The KiloCode parse-version bump only invalidates caches produced before this shared parser correction; normal caching remains enabled afterward.

Fixes #769.

Testing

• targeted provider/cache tests: 113 passed
• npm run build:cli
• npx tsc --noEmit
• git diff --check

The full suite completed with 2449 passing tests, 5 skipped tests, and 2 existing unrelated failures in tests/cache-refresh-lock.test.ts and tests/cli-durable-totals.test.ts.