ci: triage new issues with Codex

[DaleSeo]

Motivation and Context

triage.yml has been disabled with if: false since #714. This PR replaces the disabled Bash triage script with a AI-powered workflow that labels new issues. As guided in Maintain Codex account auth in CI/CD, the workflow restores auth.json from a secret, refreshes it during the run, and writes it back afterward.

This repository has two configured secrets, CODEX_AUTH_JSON and CODEX_AUTH_STORE_TOKEN. The second one is a fine-grained PAT because GITHUB_TOKEN does not have the secrets scope and cannot persist the refreshed file.

How Has This Been Tested?

Tested via workflow_dispatch against issue #1251:
https://github.com/modelcontextprotocol/rust-sdk/actions/runs/34541690342

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed