RFC-0004 M2 — advisory security and performance reviews

[cspinetta]

Implements RFC-0004 M2 (docs/rfc/0004-change-safety-gates.md) and ADR-0010.

Scope:

• Add docs/security/invariants.md — the public, sanitized distillation of the private threat model (mechanisms only).
• Add the reviewer script and skill that carry the agents' prompt and command; the skill wraps the script.
• Security review subagent: reads the diff against the invariants baseline and reasons beyond it, under the shared finding standard.
• Performance review subagent: the benchmark anchors boot and restore latency; an agent pass covers the runtime hot paths the benchmark does not measure.
• Both reviews are advisory, run locally for now, and report findings in the PR Local validation block.