feat(forge): read PR timestamps, closing refs and PR list rows (RIG-4034)

[rigel-mintaka]

This PR is part of a stack containing 6 PRs:

1. main
2. feat(store): store pull requests and their issue links (RIG-4034) #1900
3. "feat(forge): read PR timestamps, closing refs and PR list rows (RIG-4034)" (this PR)
4. feat(board): show linked pull requests on board issues (RIG-4034) #1917
5. feat(forge): link a created pull request to its issue (RIG-4034) #1926
6. feat(ingest): hydrate pull requests onto the board (RIG-4034) #1927
7. feat(ingest): retry PR hydrates held by a budget pause (RIG-4826) #1971

Summary

This is T2 of the PR-to-issue linkage design (docs/designs/server/compass-issue-model-pr-linkage-amendment/design.md, §4). It stacks on #1900. The reconciler and the projection consume these reads in T3 and T5.

• forge.PullRequest gains CreatedAt, UpdatedAt and ClosingRefs []IssueRef. Both the create response and the detail read decode the timestamps.
• pullReadQuery reads closingIssuesReferences(first: 25) behind $refs.
  • It is fetched only on the first page of a full read, and never on the checks-only read.
  • A missing connection is an error, so links are never silently cleared.
  • Hitting the cap is logged.
• ListUpdatedIssues returns UpdatedRows{Issues, Pulls}: PR rows come back in order beside the issues instead of being dropped. The reconciler still sinks only Issues.
• ListOpenPullRequests is added for the backfill pass.
• Store: PullRequestUpdatedAt, PRsBackfilledAt and MarkPRsBackfilled.

Verification

• New httptest cases cover:
  • timestamps on create and on read;
  • closing refs read on page 1 and not on page 2;
  • the checks read skipping them;
  • a GraphQL error failing the read;
  • a missing connection failing the read;
  • PR rows split from issue rows in order;
  • the open-PR list paging.
• 2 new pgtests cover the reconciler store reads.
• go test ./..., plus pgtest for store, server, ingest and forge, pass. golangci-lint reports no new findings.
• Golden fixtures carry the new fields, and created_at is canonicalized for live capture.

Risks

Every PR read now costs one extra GraphQL connection on its first page, inside a call that was already being made.

Compatibility

Internal Go only. No wire or schema change.

Documentation

None. The design record already describes the change.

Refs RIG-4034

[linear-code]

RIG-4034

[github-actions]

Compass engineering docs preview: https://compass-obs-rig-4034-t2-forg.compass-eng-docs.pages.dev

Deployed from compass-obs/rig-4034-t2-forge-reads at 5677e58.