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.

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Summary by cubic

Adds a scheduled GitHub Actions workflow to sync open PRs to Linear each day so external contributor PRs are tracked in Linear. Fulfills ENG-4902.

• New Features
  • Runs daily at 10:00 UTC and supports manual runs via workflow_dispatch.
  • Uses reusable resend/public-shared-workflows/.github/actions/sync_prs_to_linear.
  • Requires LINEAR_API_KEY and LINEAR_TEAM_ID secrets; grants write access to PRs and Issues only.

^{Written for commit f6dfbc1. 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.

_{Requires human review: Adding a new CI/CD workflow with write permissions on pull requests and issues and requiring sensitive secrets like LINEAR_API_KEY is a high-impact infrastructure change that should be reviewed by a human to ensure correct configuration and security.}