docs: consolidate to AGENTS.md and extend doc freshness check (#172)

- Replace .github/copilot-instructions.md with stub pointing to AGENTS.md
- Rewrite doc-freshness-check to audit AGENTS.md, README.md, and
  prompts/create-ado-agentic-workflow.md
- Add §7 for workflow authoring prompt validation
- Treat prompt file drift as high-priority in decision criteria
- Update rust-pr-reviewer and rust-review-command references

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesadevine