feat: add PR triage agent workflow

[nearlyforget]

Description

This PR adds the pr-triage.yml workflow to automate the initial triage of incoming pull requests. This workflow integrates with the AI-powered agent hosted in the
.github repository to ensure every contribution adheres to project standards.

Workflow Highlights:

• Real-time Triage: Triggers on opened, edited, and synchronize events.
• Guideline Enforcement: Automatically checks the PR body against the pull_request_template.md and CONTRIBUTING.md standards.
• Automated Feedback: Provides immediate feedback to authors if sections are missing or if the contribution follows all guidelines.
• Smart Labeling: Applies appropriate area and type labels based on the files changed and the PR description.

⚠️ Important Requirement:
This workflow requires the GOOGLE_API_KEY secret to be configured in the repository. It is currently waiting for the official key to be provisioned.

Category (Required)

• Core Protocol
• Governance/Contributing
• Capability
• Documentation
• Infrastructure: CI/CD, Linters, or build scripts.
• Maintenance
• SDK
• Samples / Conformance