ci: allow Argus on contributor PRs

[bokelley]

Summary

• Run Argus on pull_request_target so contributor and fork PRs can access the GitHub App and Anthropic secrets needed for review.
• Keep the workflow on the trusted base SHA and inspect PRs via GitHub APIs instead of checking out PR code.
• Add a human-review gate for PRs that modify the Argus workflow or prompt, and pin review verification to aao-ipr-bot[bot].

Validation

• `ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ai-review.yml"); puts "yaml ok"'
• npx prettier --check .github/workflows/ai-review.yml
• git diff --check
• pre-push hook: npm run typecheck and library build