[Chore] Seed demo tasks with linked pull requests

[roomote-roomote]

Opened on behalf of Daniel Riccio. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

The demo seed now creates one completed task with a linked pull request and a second completed task split across two linked pull requests. Seed tests verify all associations and reruns remain idempotent.

Why this change was made

The local demo dataset previously had no task-level pull request associations, which made it impossible to exercise or visually verify multi-PR task surfaces through the normal product path.

Impact

Fresh sandbox and preview databases now include realistic single-PR and multi-PR task examples using stable GitHub URLs. Existing demo rows remain untouched, and rerunning the seed does not duplicate associations or mark synthetic PRs as Roomote-created.

Related PRs

• Multi-PR task presentation

Validation

The seed idempotency test passes, the demo seed command successfully created all three PR associations locally, and the database package typecheck plus pre-push checks pass.

[roomote-community]

No new code issues found. See task

• packages/db/src/fixtures/seed-demo-data.ts:291 Do not mark synthetic seed associations as Roomote-created PRs.

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