fix(agt): validate Guardian decision responses

[mjatin2007]

What changed

Validates parsed, correlated Guardian decision responses against the canonical response-envelope.json schema and its method-specific AcsResult branch before the host adapter accepts them. A canonical-schema or method-result violation is audited and denied even under on_decision_failure: proceed; inability to run the local validator remains a host-side failure and follows the negotiated posture.

This follows the accepted issue's fail-closed direction, but under proceed it deliberately differs from the literal current §6.4 treatment of a malformed response. Validation covers the complete canonical response envelope, not only deny.reasoning. This draft should not be treated as conformant until §6.4 is clarified, and it should not merge ahead of that clarification unless maintainers direct otherwise. Response-signature verification is separate and remains pending in #229.

Which issue does this implement

Part of #182

• This is an editorial correction (typo, grammar, link, formatting) with no change in
  meaning, so it needs no issue

Base branch

• integration, because this touches the specification, schemas, a reference
  implementation, an adapter, tests, or CI
• main, because every changed path is on the documentation lane allowlist in
  CONTRIBUTING.md

Type of change

• Specification change (schema, hooks, events, AgBOM)
• Reference implementation or adapter
• Documentation
• Tooling or CI
• Governance (licensing, security policy, contributor docs)

I tested this

• I synced my branch with the base branch before opening this
• uv run pytest -v passes on my machine
• uv run mkdocs build --strict passes on my machine

The AGT validation also passes on the repository's pinned Bun 1.3.11: TypeScript succeeds, and the complete Bun suite reports 1,123 passed, 1 skipped, 0 failed across 68 files.

Checklist

• Commits are signed off with git commit -s (required by the DCO)
• Prose follows STYLE.md
• No secrets, tokens, or internal URLs in the diff

Security

• This change has no security impact

Security impact: schema-invalid or method-invalid Guardian responses are blocked and audited. Local validator unavailability remains posture-governed and is attributed to the host rather than the Guardian.