docs(phase-4): add docs-code consistency tests and final polish

Part of #224 — v2 docs rewrite (Diátaxis).

## Goal

Drift-proofing + final QA before merge.

## Tasks

- [ ] Add `tests/cli/test_docs.py` (openrpc ↔ api.md, typer ↔ cli.md, config env vars ↔ cli.md, link checker)
- [ ] Update llmstxt config + regenerate `llms.txt` / `llms-full.txt`; verify with an LLM client
- [ ] Run `make format` + `make quality`; fix mdformat issues
- [ ] Full docs review pass: every page read for tone consistency (Diátaxis: no quadrant mixing)

## Dependencies

- Blocked by: #226 (Phase 1 — reference), #228 (Phase 3 — how-to)
- Blocks: Phase 5 (ship)

## Done when

- [ ] CI green with the new docs test
- [ ] `llms.txt` regenerated and LLM-verified
- [ ] Zero mdformat/quality failures