feat(store): store pull requests and their issue links (RIG-4034)

[rigel-mintaka]

This PR is part of a stack containing 5 PRs:

1. main
2. "feat(store): store pull requests and their issue links (RIG-4034)" (this PR)
3. feat(forge): read PR timestamps, closing refs and PR list rows (RIG-4034) #1914
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

Summary

This is the first build slice of the PR-to-issue linkage design (docs/designs/server/compass-issue-model-pr-linkage-amendment/design.md, Plan T1). Nothing calls the new code yet: T2 and T4 wire it in.

• Proto: CreatePullRequestRequest.issue = 7, carrying a new PullRequestIssueLink message. Go and agent TS are regenerated.
• Migration 0003_pull_request_links.sql:
  • Adds the pull_requests table, with the PR stored whole as JSONB.
  • Adds pull_request_issue_links (source 1 = explicit, 2 = closing ref) and its issue index.
  • Adds forge_repo_subscriptions.prs_backfilled_at.
  • Both new tables are tenant-scoped: ENABLE and FORCE RLS with the fail-closed policy and explicit grants. A set_updated_at trigger is created on pull_requests.
• Store:
  • CreatePullRequestWithLink writes the ownership row, the PR row (an existing hydrated row wins) and the explicit link in one transaction.
  • UpsertPullRequest is guarded by recency and replaces closing refs. It returns the issues linked before and after the write.
  • PullRequestsForIssues returns PRs ordered by forge creation time. A PR attaches to an issue through a closing ref only when none of its explicit targets is on the board.
  • FallbackIssuesForTarget.
  • GitHub repos are lowercased at the store boundary. The ownership row keeps the caller's casing so the self-origin lookup still matches.

Verification

• 10 new pgtests pass, including the 9 that the design names for T1.
• Each mutation probe turned its test red: no lowercasing, no older-update skip, old links left out of affected, and no explicit link.
• go test -tags pgtest ./internal/store/... ./server/... ./internal/ingest/... and the non-pgtest store suite pass.
• golangci-lint reports no new findings; sqlc generate and buf generate are clean.

Risks

This is a new append-only migration. It adds tables, plus one nullable column on an existing table, and changes no existing rows.

Compatibility

The proto field is additive. Old clients leave issue unset.

Documentation

None. The design record already describes the change.

Refs RIG-4034

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[linear-code]

RIG-4034

[github-actions]

Compass engineering docs preview: https://compass-obs-rig-4034-t1-pr-s.compass-eng-docs.pages.dev

Deployed from compass-obs/rig-4034-t1-pr-store at be700a4.

[rigel-mintaka]

Back to draft: migration 0003 collides with five open PRs ahead of it in the slot order (#1513, #1752, #1595, #1859, #1602). This PR will be rebased and renumbered when its turn comes.