chore: add sync-prs-to-linear action

[dielduarte]

Adds the sync-prs-to-linear workflow to automatically sync open external contributor PRs to Linear daily.

---

Summary by cubic

Adds a GitHub Actions workflow to sync open external-contributor PRs to Linear daily at 10:00 UTC, with manual trigger support. Implements ENG-4902 to keep Linear updated without manual triage.

• Migration
  • Ensure repository secrets LINEAR_API_KEY and LINEAR_TEAM_ID are set.

^{Written for commit 0e3f704. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: This change adds a new GitHub Actions workflow to sync open PRs to Linear, which is a low-risk, self-contained CI configuration that does not affect application logic, data, or critical infrastructure.}