Add a short guide for validating evidence integrity after an evaluation

[MuttleyD]

Context

MIDAS records an evidence envelope for every governed evaluation. The control-plane example walkthrough already shows how to run evaluations, but new users may not immediately understand what to do with the returned envelope_id.

A short guide or section explaining how to inspect and verify evidence would help users understand one of the most important MIDAS capabilities.

Scope

Add a concise evidence validation section to docs/examples/control-plane.md.

The section should explain:

• /v1/evaluate returns an envelope_id;
• the envelope records the submitted request, resolved authority, outcome, and audit evidence;
• users can retrieve the decision by request ID/source;
• users can check the envelope integrity endpoint;
• valid: true means the stored audit chain matches the envelope integrity metadata;
• evidence integrity verification does not require application secrets.

Include example commands for:

• retrieving a decision by request ID/source;
• checking evidence integrity;
• retrieving the evidence packet, if already documented.

Acceptance Criteria

• docs/examples/control-plane.md clearly explains what to do with an envelope_id.
• The evidence integrity check is easy to find after the evaluation examples.
• Example commands use the local quickstart tokens and example request IDs.
• The wording does not overclaim cryptographic guarantees beyond the current implementation.
• Links to docs/core/envelope-integrity.md where deeper explanation is needed.
• No runtime behaviour is changed.
• No real secrets are added.
• git diff --check passes.

Out of Scope

• Changing the evidence envelope schema.
• Changing audit-chain behaviour.
• Adding new evidence endpoints.
• Adding automation or scripts.
• Changing runtime evaluation behaviour.