feat(forge): link a created pull request to its issue (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) #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)" (this PR)
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 T4 of the PR-to-issue linkage design (docs/designs/server/compass-issue-model-pr-linkage-amendment/design.md, §1 and §3). It stacks on #1917.

• create_pull_request takes an optional issue link. The server checks only its shape and never reads the issue:
  • number 0 is rejected with invalid_argument;
  • an unresolvable forge is rejected with not_found;
  • a blank repo on another forge is rejected with invalid_argument.
• An unset or blank repo on the PR's forge means the PR's repo.
• An UNSPECIFIED forge means the PR's forge.
• The ownership row, the PR row and the explicit link are written in one transaction:
  • the PR row keeps the forge's created_at;
  • its forge_updated_at is the epoch, so the next hydrate always wins.
• IssueProjection.PublishExplicitLink then republishes the target issue, plus the issues whose closing-ref fallback it took over.
• A memo hit writes nothing.
• The agent tool forge_create_pull_request gains issue (selector + repo + number).
• The manager and owner role prompts tell agents to pass it.

Verification

• Go unit tests cover link mapping (including UNSPECIFIED forge and blank repo), each shape error, and the memo hit. Each test was mutation-probed red.
• A pgtest checks that a linked create shows on the board issue at once with forge_created_at stored, and that a later hydrate replaces the row.
• TS tests cover the wire mapping and the schema.
• go test ./..., pgtests, golangci-lint, bun test, tsc and biome all pass.

Risks

A failed publish after the commit is only logged; the next issue hydrate heals the cache.

Compatibility

CreatePullRequestRequest.issue is additive (it lands in #1900).

Documentation

packages/compass-agent/AGENTS.md documents the new tool parameter.

Spec-impact: none. Refs RIG-4034

[linear-code]

RIG-4034

[github-actions]

Compass engineering docs preview: https://compass-obs-rig-4034-t4-crea.compass-eng-docs.pages.dev

Deployed from compass-obs/rig-4034-t4-create-link at 755bca7.

Changed pages:

• packages/compass-agent/AGENTS.md