CI: add GitHub Actions test workflow (3.11/3.12/3.13) + branch protection

[ernestprovo23]

No CI exists (.github/workflows absent). Sibling project mcp-warden is explicitly CI-first (164 tests gated); conclave should match. The suite is fully offline (mocks post_json/call_model, no keys/network), so CI is cheap and deterministic.

Scope

• .github/workflows/ci.yml on push + pull_request.
• Matrix: Python 3.11 / 3.12 / 3.13 (pyproject requires-python = ">=3.11"; suite verified on 3.13). Ubuntu.
• Install: pip install -e ".[dev]".
• Run: pytest -q (config already in pyproject: asyncio_mode=auto, testpaths=["tests"]). Use -p no:cacheprovider.
• Pair with branch protection (require CI green to merge), matching mcp-warden.

Acceptance

• ci.yml runs pytest green on the 3.11/3.12/3.13 matrix on every PR; a failing test blocks merge.

Source: pre-dev planning audit 2026-06-08 (finding C-1).

[ernestprovo23]

Done — GitHub Actions test workflow (matrix 3.11/3.12/3.13) + branch protection landed in PRs #23 and #27. Required checks active on main.