feat(skills): Automate PR triage and CLA verification

Introduce the adk-pr-triage skill and its supporting triage_pr.py helper
script to guide AI coding assistants through conducting rigorous and
automated triage of GitHub pull requests.

The workflow script handles compliance verification, validates that the
contributor's Google CLA signature status check run is SUCCESS, fetches
remote branches, and automates PR updates via server-side rebase before local checkout.

Provides an interactive 'Local Review' option that integrates branch checkout,
preserves original PR commit metadata, and triggers /adk-review for rigorous
quality control before squashing and pushing changes to Gerrit.

Change-Id: If311b90b185636f4f737aa04a3c50a4a460b94db

Author: boyangsvl