resources-graph: touch-driven refresh and rate-limit handling

[antejavor]

Part of PRD: resources-graph. Vocabulary: context-graph/resources-graph/CONTEXT.md. Prototype reference (throwaway): branch prototype/github-resource-graph-model.

What to build

Freshness and rate limits.

• Touch-driven revalidation: a real re-fetch Touch of a stored Resource makes the next Sweep fetch it again (conditional request for items, since=<fetched_at> for listings). Cache Reads never schedule refresh. No TTL, no crawl.
• No-token path: REST within the 60-calls-an-hour budget; exhaustion → Unresolved rate_limited, retried by a later Sweep.
• Verify during this slice: does an issue keep its node_id after transfer? Record the finding on the PRD.

Acceptance criteria

• Re-fetching a stored issue updates it on the next Sweep; a Cache Read does not.
• Rate-limited Touches are retried and resolve once budget returns.
• Reaction-only changes are documented as not refreshing (updated_at doesn't move).