[codex] chore(ci): add a narrow architecture contract gate

[DinhLucent]

Summary

• replace the broad multi-version CI run with a narrow reviewed contract gate
• add a focused architecture-boundary and contract-smoke test suite
• keep the gate aimed at the specific runtime contracts reviewed in the modernization wave

Why

The full matrix run is expensive and noisy for the current repository shape. This follow-up narrows CI to the architectural contracts we explicitly hardened, so regressions in those boundaries are caught earlier and more intentionally.

Dependency note

This PR depends on the runtime contract surfaces introduced in #205. It is intentionally a draft until that work lands, because tests/test_ci_contract_gate.py exercises the new Document and AppConfig behaviors added there.

Impact

• CI focuses on a smaller Python 3.12 gate instead of a broad matrix
• reviewed architecture boundaries become executable checks
• the change is governance-oriented rather than feature-oriented

Validation

• the gate was exercised in the implementation flow with focused pytest and ruff checks