Add GitHub Actions workflow to auto-close stale issues and PRs

[kmatsunami]

Motivation

• Reduce repository noise by automatically marking and closing issues and pull requests that have been inactive for an extended period.

Description

• Add .github/workflows/close-stale.yml which uses actions/stale@v9 to mark and close stale issues and PRs.
• Configure the workflow to run on a daily schedule (cron: "30 3 * * *") and via manual workflow_dispatch.
• Mark items stale after 30 days and close them after 7 more days, with custom stale/close messages, stale labels, and exempt labels for issues and PRs.
• Grant the workflow permissions issues: write and pull-requests: write so it can modify and close issues/PRs.

Testing

• Validated the workflow YAML structure by inspection, which succeeded.
• Committed the new file and verified the repository state with git status and git show, which succeeded.
• No workflow run was executed as part of this change.

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Codex Task