Migrate AI commit message generation to Copilot CLI

[dblock]

Problem

GitHub Models (models.github.ai, the backend actions/ai-inference@v1 used for AI commit summaries in #87) was fully retired on July 30, 2026:

Error: 410 GitHub Models is temporarily unavailable as part of a scheduled retirement brownout.

This is permanent, not a temporary outage — every scheduled update.yml run will fail from now on.

Fix

actions/ai-inference was rewritten (v3, released July 29, 2026) to exclusively shell out to the GitHub Copilot CLI instead of calling GitHub Models. This PR updates the workflow accordingly:

• Installs and authenticates the Copilot CLI (npm install -g @github/copilot) before invoking actions/ai-inference@v1, using COPILOT_GITHUB_TOKEN sourced from a new COPILOT_PAT repo secret.
• Drops the now-unnecessary models: read permission.
• The v3 action also dropped structured JSON schema output (responseFormat/jsonSchema), so the prompt now asks for a bare JSON object with no markdown fences, and the "Build commit message" step strips any ``` fences defensively before parsing with jq.

⚠️ Action required

This requires adding a COPILOT_PAT secret (a PAT for an account with Copilot CLI access) to this repository's Actions secrets for the workflow to actually run. Copilot CLI usage is free for individual accounts (with monthly limits) and open source maintainers can request higher/unlimited access — see https://github.com/features/copilot/plans.

[dblock]

I got a COPILOT_PAT under my dblock username (should be free at least while I work at MSFT)