Upgrade AutoTriage to v5

[danielchalmers]

Moves AutoTriage from v4 to v5, switches it from Gemini to Anthropic, and widens what it triages.

• What it triages:
  • Before: opened issues and PRs.
  • After:
    • new and reopened issues;
    • pull requests when they're opened, reopened or marked ready for review, skipping drafts;
    • new comments, skipping bots.
• When it acts:
  • New activity: triaged live.
  • A manual run from the Actions tab: a blank issues means the whole backlog, and it only plans unless you untick dry-run.
• Model: uses ANTHROPIC_API_KEY, which defaults to claude-haiku-5-5. The Gemini key is no longer mapped.
• No checkout: actions/checkout is gone, because v5 reads the policy and README through the API.
• Artifacts: each run uploads artifacts/, with its prompts and each plan, for review.