Adds rulespec.yaml and envelope.yaml support for machine-readable
invariant checking during plan completion.
- Add invariants module with Rulespec, ActionEnvelope, and evaluation logic
- Add Invariants section to system prompt with workflow instructions
- Show rulespec/envelope file status in plan verification output
- Rulespec written during planning (captures constraints from task)
- Envelope written after implementation (documents what was built)
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