Add invariant extraction template

[Alan-Jowett]

Summary

Add a template that extracts structured invariants (MUST/SHOULD/MAY constraints, state transitions, timing assumptions, error conditions) from existing specifications or source code. This is Phase 3 of the specification integrity roadmap.

Motivation

Invariant extraction is foundational for two use cases:

1. Feeding audit templates — extracting structured requirements from an RFC or legacy spec so they can be audited with audit-traceability or audit-code-compliance
2. Standalone analysis — understanding what a spec or codebase actually guarantees, in a machine-readable format

Scope

New components needed:

• Template: extract-invariants — consumes specification text or source code; produces a structured invariant set (requirements-document format with MUST/SHOULD/MAY classification)
• Protocol (optional): May need a thin invariant-extraction reasoning protocol, or may be able to compose requirements-elicitation + requirements-from-implementation depending on whether input is spec or code

Reused components:

• specification-analyst or reverse-engineer persona (depending on input type)
• requirements-doc format (invariants are structured requirements)
• anti-hallucination + self-verification guardrails

Open questions

• Should this be one template handling both spec and code inputs, or two separate templates?
• Should the output format be requirements-doc or a new invariant-set format with additional structure (e.g., state machine notation)?
• How does this relate to the existing reverse-engineer-requirements template? Is this an extension or a separate concern?

Acceptance criteria

• Template (and protocol if needed) created
• Manifest updated
• tests/validate-manifest.py passes
• Scenario added to docs/scenarios.md