docs: accuracy sweep, architecture restructure, two ADRs, review skill

- AGENTS.md Commands: fix typecheck/build descriptions; add lint, lint:fix,
  test:coverage, knip, publint, hygiene (were undocumented).
- Drop the bare `yarn demo` for explicit `demo:echo` + `demo:coding`; update
  README, examples READMEs (and document coding-agent in examples/README).
- New cookbook guide: adding-a-vendored-package.md (the missing "add" half of
  vendor/README's update-only procedure).
- architecture.md: add a table-of-contents and extract the Extension cookbook
  to docs/cookbook/extension-cookbook.md (link-preserving); drop the completed
  "restructure this document" TODO.
- ADR 0009 (capability seams) + 0010 (twin LLM adapters), and a "when to write
  an ADR" standard in adr/README.
- Add a committed dsh-code-review skill under .agents/skills, exposed to Claude
  Code via a tracked .claude/skills symlink (gitignore carve-out).

.gitignore

@@ -6,7 +6,6 @@ lib/
 .yarn/
 yarn-error.log
 examples/*/*.jsonl
-.claude/
 coverage/
 .vscode/
 .DS_Store