Generate AI commit summaries for automated API updates

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Problem

The daily update workflow (.github/workflows/update.yml) commits scraped API changes with a generic, date-only message like Updated from Slack docs, 2026/08/11 (see ff2d6a0). This tells you nothing about what actually changed without opening the diff.

Fix

Add an AI-generated commit summary step to the workflow, using only GitHub Actions (no extra Ruby code):

• New .github/prompts/commit-message.prompt.yml: a structured prompt for actions/ai-inference@v1 that asks a model to turn a git diff / git diff --stat into a short commit title (<=72 chars, imperative mood) and a summary of 1-5 bullet points. Uses responseFormat: json_schema so the response is guaranteed valid, parseable JSON.
• update.yml changes:
  • Adds permissions: contents: write / models: read (required for GitHub Models inference; no extra secrets or Copilot subscription needed, it uses the default GITHUB_TOKEN).
  • Checks whether rake api:update produced any changes; if not, skips the AI call entirely.
  • Captures git diff --stat and a truncated git diff (20,000 chars) to files and passes them into the prompt via file_input.
  • Runs actions/ai-inference@v1 with the prompt file, parses title/summary out of the JSON response with jq.
  • Falls back to the original generic, date-based message if there are no changes, the AI step fails, or the response is malformed/empty, so the workflow never blocks on this step.
  • Passes the resulting multi-line message to the existing EndBug/add-and-commit@v9 step, unchanged otherwise.

Example resulting commit message

Add chat.appendStream and chat.stopStream methods

- Added chat.appendStream and chat.stopStream methods
- Added metadata argument to chat.postEphemeral
- Marked send_on_behalf_not_allowed error on chat.postMessage

Verified .github/workflows/update.yml and .github/prompts/commit-message.prompt.yml parse as valid YAML, and that the embedded jsonSchema is valid JSON.