Merge bitcoindevkit/bdk#2259: chore(ci): add stale issue triage workflow

Merge bitcoindevkit/bdk#2259: chore(ci): add stale issue triage workflow

5edb32302e5fd6a7e3e258980ca4d4cfd684e5d9 chore(ci): add stale issue triage workflow (Rishit Modi)

Pull request description:

  ### Description

  Follow-up to bitcoindevkit/bdk_wallet#497.

  Adds `.github/workflows/stale.yml`, based on the workflow introduced in the referenced PR, to automatically label and close inactive issues and abandoned pull requests in this repository.

  The workflow uses `actions/stale@v11`

  - Issues inactive for 180 days -> marked stale -> closed after 30 additional days
  - Pull requests inactive for 180 days -> marked stale -> closed after 30 additional days
  - Issue exemptions: `release`, `security`, `audit`, `do-not-close`, and `good first issue`
  - Pull request exemptions: `blocked`, `security`, and `do-not-close`
  - Issues and pull requests associated with milestones are exempt
  - Runs weekly on Mondays at 09:00 UTC
  - Pull request runs are automatically forced into dry-run mode for safe validation
  - Stale labels are removed when new activity occurs

  The timing and exemption policy are initially kept consistent with bitcoindevkit/bdk_wallet#497. They can be adjusted based on feedback.

  ### Notes to the reviewers

  The workflow runs with `debug-only: true` on pull request events. I also recommend running it manually with `debug_only=true` after merging and reviewing the results before the first scheduled Monday run.

  ### Changelog notice

  ci: add automated stale issue and pull request triage

  ### Checklists

  #### All Submissions:

  - [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  notmandatory:
    ACK 5edb32302e5fd6a7e3e258980ca4d4cfd684e5d9

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