docs(phase-0): audit docs and scaffold the new structure

Part of #{{MAIN}} — v2 docs rewrite (Diátaxis).

## Goal

Inventory of every factual claim and its source of truth; new directory skeleton in place.

## Tasks

- [ ] Diff docs vs main to harvest v2 deltas already written
- [ ] Create `docs/how-to/`, `docs/reference/`, `docs/explanation/` (no section index pages needed)
- [ ] Update `mkdocs.yml` nav + llmstxt sections; run local `mkdocs serve` to validate skeleton
- [ ] Extract CLI help text and openrpc method list as the audit baseline
- [ ] Adopt the writing style & first-mention linking rules — agree before writing starts

## Dependencies

- Blocked by: nothing
- Blocks: Phase 1 (reference), Phase 2 (explanation), Phase 3 (how-to), Phase 4 (automation), Phase 5 (ship)

## Done when

- [ ] Skeleton builds locally (`mkdocs serve` clean)
- [ ] Audit baseline extracted (CLI help output, openrpc method list)
- [ ] Style rules agreed

Start by diffing the current docs against main and reviewing mkdocs.yml, then inspect the existing CLI help output and openrpc method list as the audit baseline. Create the named documentation directories, update the navigation and llmstxt sections, and run mkdocs serve locally. Done means the skeleton builds cleanly, the audit baseline is extracted, and the writing and first-mention linking rules are agreed.

Written by the indexing model from the issue text.