chore: Add stale agent auditor workflow

[nearlyforget]

Description

This pull request adds the ADK Stale Agent Auditor workflow to automate repository health maintenance. It identifies and manages inactive issues to keep the backlog clean and actionable.
Key Features:

• Automated Auditing: Runs daily (at 06:00 UTC) via a scheduled GitHub Action.
• Configurable Thresholds:
  • Stale Threshold: 72 hours of inactivity.
  • Close Threshold: 96 hours of further inactivity.
• AI-Powered Analysis: Utilizes the ADK Stale Agent with gemini-2.5-flash.
• License Compliance: Includes the standard Apache 2.0 license header.

Implementation Details:
The workflow is added as .github/workflows/stale-agent-auditor.yml. It uses dynamic GitHub context variables to ensure the configuration is identical and portable across all organization repositories.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Checklist

• I have followed the Contributing Guide.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.