fix(agents): enforce permanent canonical agent ordering with regression tests

The canonical agent order is: sisyphus → hephaestus → prometheus → atlas

Changes:
- Export CANONICAL_CORE_AGENT_ORDER as single source of truth
- Sort non-core agents alphabetically for deterministic ordering
- Add 12 regression tests including 100-permutation stability test

This permanently resolves the agent ordering saga that caused 15+ commits,
8+ PRs, and multiple reverts due to ZWSP prefixes, Object.entries() order
dependencies, and inconsistent merge sequences.

Closes #3281