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).

docs/cookbook/adding-a-vendored-package.md

@@ -0,0 +1,56 @@
+# Cookbook: adding a vendored package
+
+When the harness needs another upstream Cordis package (e.g. `@cordisjs/plugin-http`), it is **vendored** as pinned source under `vendor/`, not added as an npm dependency — see [ADR 0001](../adr/0001-vendor-cordis-as-source.md) for why. [vendor/README.md](../../vendor/README.md) covers *updating* an already-vendored package; this guide is the file-by-file checklist for adding a **new** one. (Verified against the existing vendored set; if it drifts, fix it here.)
+
+## 1. Copy the source in
+
+```
+vendor/<dir>/
+  package.json     # from upstream; set "private": true, keep name/exports/type
+  tsconfig.json    # extends ../../tsconfig.base.json (see shape below)
+  src/             # the upstream src/ verbatim
+  README.md LICENSE # if upstream ships them
+```
+
+`tsconfig.json` mirrors the other vendored packages — `rootDir: src`, `outDir: lib`, the strictness relaxations upstream code needs, and a `references` entry for every other vendored package it imports:
+
+```jsonc
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src", "outDir": "lib",
+    "noUncheckedIndexedAccess": false, "exactOptionalPropertyTypes": false,
+    "noImplicitOverride": false, "noUnusedLocals": false, "noUnusedParameters": false
+  },
+  "include": ["src"],
+  "references": [{ "path": "../cordis" }, { "path": "../cosmokit" }]
+}
+```
+
+`package.json` invariants: `"private": true` (vendored packages are never published), keep upstream's `name`/`version`/`exports`/`type`, and list its cordis deps in `peerDependencies` (matching the upstream manifest). Transitive upstream deps must themselves be vendored or already present — vendoring one package often means vendoring its dependency tree (e.g. `@cordisjs/plugin-http` pulls `@cordisjs/fetch-file`).
+
+## 2. Register it in the root configs
+
+| File | Change |
+|---|---|
+| `tsconfig.base.json` | add `"<npm-name>": ["./vendor/<dir>/src"]` to `paths` |
+| `tsconfig.typecheck.json` | add `"<npm-name>": ["./vendor/<dir>/lib"]` — this file points at built declarations, not src. If the package's `types` entry isn't `lib/index.d.ts`, point at that built file instead (e.g. `logger-console` maps to `./vendor/logger-console/lib/shared`, matching its `"types": "lib/shared.d.ts"`). |
+| `tsconfig.build.json` | add `{ "path": "./vendor/<dir>" }` to `references` (before the `packages/*` entries) |
+| `vendor/README.md` | add a manifest table row (dir, npm name, version, upstream repo, commit SHA) and log any local modifications |
+| `scripts/publint-all.ts` | only if the vendored package is itself published from here (vendored deps normally are not — skip) |
+
+Covered automatically by globs — no edits needed: root `package.json` workspaces (`vendor/*`), `tsdown.config.ts`, `vitest.config.ts`, `eslint.config.mjs`. A per-package `vendor/<dir>/tsdown.config.ts` is needed ONLY if the build shape diverges from the root default (dual ESM/CJS or multiple entries — see `vendor/schemastery` and `vendor/logger-console`).
+
+## 3. Mind the manifest guard
+
+`scripts/check-vendor-manifest.sh` (a pre-commit hook) fails if anything under `vendor/*/src` is staged without `vendor/README.md` also staged. Stage the manifest update alongside the source so the commit passes.
+
+## 4. Verify
+
+```sh
+yarn install        # registers the workspace
+yarn typecheck      # the base→lib path split means: run once after a fresh add
+yarn build && yarn test && yarn constraints
+```
+
+Note the `tsconfig` two-map split (called out in [AGENTS.md](../../AGENTS.md) § Secrets/.env): `lint`'s type-aware rules resolve vendored packages through their built `lib/` declarations, so run `yarn typecheck` (which builds them) once after adding the package or lint reports unresolved-type errors.