mirror-history: docs (reference page, action catalogue, reserved tag) #162

Part of #157.

Document the mirror-history feature once implemented.

Scope

• Finalize docs/architecture/acquire/mirror-history.md (drop the "proposed" status once shipped).
• Add a mirror-history reference page under docs/reference/ (artifact format, media types, annotations, blob schema, retrieval commands).
• Add the mirror-history action to the action catalogue (docs/reference/workflow-actions.md).
• Document the reserved mirror-history tag (never a valid upstream tag to mirror) and the new mirror job-summary outcome.
• Update docs/contributing/workflow-naming.md if a new verb/category is warranted; ensure the acquire index links are complete.

Depends on: #158, #159 (and #160/#161 for the dashboard behaviour).