Add CI and ownership

[nikki-t]

P1-13 — Add CI and ownership.

• Add .github/workflows/cit-self-test.yml: install, ruff check, pytest, schema-drift, and rules-drift.
• Add a CODEOWNERS file for contracts/* and rules/*.

AC:

• Green on a trial PR.
• A stale committed artifact fails CI.

[nikki-t]

Tracked in beads as confluence-contracts-6c8 (Phase 1, plan: #5). Sub-tasks and dependency state live there; this issue stays the team-visible record and closes when its PR merges.