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/extension-cookbook.md

@@ -0,0 +1,48 @@
+# Cookbook: extension plugin shapes
+
+The three plugin shapes you write against the harness extension surface, as illustrative snippets (elided imports and helper stubs — not copy-paste-complete). For the full step-by-step guides see [adding a package](./adding-a-package.md), [adding a tool](./adding-a-tool.md), and [adding an LLM adapter](./adding-an-llm-adapter.md); for the seams these hook into see [docs/architecture.md](../architecture.md).
+
+## A tool plugin
+
+A tool registers on `ctx.tools`. The annotated `defineTool` example (typed `execute` args, result shaping, the `run_in_background` pattern) lives in [adding-a-tool.md](./adding-a-tool.md) — that guide is the source of truth for the tool shape. Raw JSON-Schema `ToolDefinition`s are also accepted by `ctx.tools.register()` directly (that is how MCP-sourced tools arrive); `defineTool` is the typed sugar for first-party tools.
+
+## A hook plugin (permission gate)
+
+A hook wraps the `tools/execute` waterfall to veto or rewrite a call — the seam where sandbox, permission, and plan-mode plugins live.
+
+```ts
+export const name = 'permission-gate'
+
+export function apply(ctx: Context) {
+  ctx.on('tools/execute', async (exec, next) => {
+    if (!(await isAllowed(exec))) {
+      return {
+        callId: exec.callId,
+        content: [{ type: 'text', text: 'Denied by policy.' }],
+        isError: true,
+      }
+    }
+    return next()
+  })
+}
+```
+
+## A UI plugin
+
+A UI plugin consumes `agent/stream-chunk` and session events for rendering, and drives input back in via `agent.send()` / `agent.steer()`.
+
+```ts
+export const name = 'my-ui'
+export const inject = ['agents']
+
+export function apply(ctx: Context) {
+  ctx.on('agent/stream-chunk', (agent, turn, step, chunk) => {
+    if (chunk.type === 'text-delta') render(chunk.text)
+  })
+  onUserInput(text => ctx.agents.get('main')?.send([{ type: 'text', text }]))
+}
+```
+
+## Runnable wirings
+
+Two complete examples load their plugin trees from `cordis.yml` with HMR: [`examples/echo-agent`](../../examples/echo-agent) (mock model + echo tool — the all-mock skeleton check, `yarn demo:echo`) and [`examples/coding-agent`](../../examples/coding-agent) (DeepSeek V4 + the bash tool suite — the real thing, `yarn demo:coding`).