Add a scheduled fixture-freshness audit comparing state against last-reviewed timestamps

[knytcomics-ui]

Problem

Ground-truth labels can go stale — a "suspicious" pattern documented today might, per further investigation, turn out to have an innocent explanation, or new information might upgrade a label's confidence. Nothing currently tracks when a fixture was last reviewed, so there's no way to know which entries are due for a re-look versus which were added yesterday.

Requirements

• Add an optional last_reviewed (ISO 8601 date) field to each destination (coordinate with the schema-formalization issue).
• Add a scheduled GitHub Actions workflow (e.g. monthly, via on: schedule) that scans destinations.json for entries with no last_reviewed date or one older than a configurable threshold (propose 180 days), and opens (or updates) a single tracking issue listing which destinations are due for review — rather than spamming one issue per stale fixture.
• Ensure the workflow is idempotent (updates the existing tracking issue's body rather than opening duplicates on repeated runs).

Acceptance Criteria

• Workflow runs on schedule and correctly identifies stale/missing-date entries against a test fixture set with deliberately old dates.
• Only one tracking issue is ever open at a time for this purpose, kept up to date.

Part of the Gryd-lock organization. This repo supplies the labelled testnet fixtures and stub scores consumed by grydlock-oracle-adapter's StubOracle and evaluated end-to-end by grydlock-research.