Add examples/coding-agent and the docs cookbook

The first real agent wiring: DeepSeek V4 + the bash tool suite + stdio
chat + JSONL persistence, runnable via yarn demo:coding (reads the
gitignored repo-root .env through process.loadEnvFile).

- examples/coding-agent: cordis.yml wiring both real plugin families
  (llm-deepseek with !!js env secrets; bash-local + tool-bash), a
  bash-only coding system prompt, a max-steps-guard plugin (bounds
  runaway turns via the agent/turn-continuation waterfall — abort()
  from step-end is a no-op by then), and a stdio UI with dimmed
  reasoning and exit-on-idle for piped stdin.
- e2e (yarn test:e2e, key-gated): full-loop.e2e.ts runs a real model
  against the real bash tool; coding-task.e2e.ts is the swebench-style
  smoke — the model fixes a buggy add.js in a temp dir and the test
  re-runs node add.test.js itself rather than trusting the agent.
- docs/cookbook: adding-a-package (the verified checklist),
  adding-a-tool (execute() contract, background pattern, seams),
  adding-an-llm-adapter (protocol obligations, mock-server testing,
  e2e policy). AGENTS.md layout/commands/secrets sections updated;
  architecture.md points at both examples and the cookbook.
- vitest.e2e.config.ts: serialize test files + retry twice — parallel
  e2e files trip the shared internal key's concurrency quota.
- fix: the !js YAML tag spelling in docs/JSDoc is actually !!js
  (js-yaml resolves custom tags under tag:yaml.org,2002:js).

docs/cookbook/adding-an-llm-adapter.md

@@ -0,0 +1,75 @@
+# Cookbook: adding an LLM adapter
+
+How to connect a new model provider. Reference implementations:
+`packages/llm-deepseek` (hand-rolled HTTP/SSE) and `packages/llm-pi-ai`
+(wrapping an LLM library). Read the `StreamChunk` doc in
+`packages/llm/src/types.ts` first — it records the protocol conventions both
+adapters were verified against.
+
+## The shape
+
+```ts
+class MyAdapter extends LlmAdapter {
+  async * stream(options: GenerateOptions): AsyncIterable<StreamChunk> { … }
+}
+
+export const name = 'llm-myprovider'
+export const inject = ['llm']
+export const Config: z<Config> = z.object({ apiKey: z.string(), … })
+
+export function apply(ctx: Context, config: Config) {
+  ctx.llm.registerAdapter(['model-a', 'model-b'], new MyAdapter(…))
+}
+```
+
+Registration is effect-based (HMR-safe); one adapter per model name —
+duplicates throw. Secrets are cordis-native: schemastery Config with env
+fallbacks, fed from cordis.yml via `!!js process.env.MY_KEY`. Never read
+ad-hoc key files in code.
+
+## Protocol obligations (the contract two implementations verified)
+
+- Emit `usage` BEFORE `finish`; emit NOTHING after `finish`. The robust way:
+  buffer finish/usage until the provider's end-of-stream marker, then flush
+  (handles providers that send trailing usage-only chunks).
+- Tool-call `arguments` are RAW JSON strings end-to-end; stream fragments as
+  `argumentsDelta`. If your provider hands back parsed objects, re-stringify
+  at `block-end`.
+- Allocate block `index`es in first-seen stream order; reuse the index for
+  every delta of the same block.
+- Errors have exactly two sanctioned paths: THROW from `stream()` (transport
+  and protocol failures — use `LlmError` with a stable code), or end the
+  stream with `finish {kind: 'error' | 'aborted'}` (provider in-band
+  failures). Consumers handle both; pick per failure class and document it.
+- Honor `options.signal` (pass it to fetch / your SDK).
+- `prefill` and other unsupported `GenerateOptions` fields: throw
+  `LlmError(..., 'UNSUPPORTED')` rather than silently dropping.
+
+Provider-specific request knobs (thinking modes, effort levels) belong in
+the ADAPTER's Config, not in `GenerateOptions` — the core vocabulary stays
+provider-neutral.
+
+## Structure that worked
+
+Split the adapter into testable stages (llm-deepseek's layout): wire types
+(`types.ts`, coverage-exempt) → request serializer → SSE/transport parser →
+chunk-translation state machine → a thin adapter class wiring them. Each
+stage gets its own unit suite.
+
+## Testing
+
+- **Unit: mock the provider, not the harness.** A scripted `node:http`
+  server speaking the provider's wire format covers happy paths, every error
+  status, malformed payloads, premature closes, and aborts — no network, and
+  it drives the 100% per-file coverage gate. Works for SDK-backed adapters
+  too (point the SDK's baseURL at the mock).
+- **Hostile framing tests.** Split stream payloads at arbitrary byte
+  positions (including mid-UTF-8) — real networks do.
+- **E2E: `tests/*.e2e.ts`** under `yarn test:e2e`, gated with
+  `describe.skipIf(!process.env.MY_KEY)` so CI (no secrets) stays green.
+  Cover each model × each provider mode you map (thinking on/off, effort
+  levels), a tool-call round trip INCLUDING the follow-up turn with results
+  in history, and loose assertions only (substring/structure, bounded
+  maxTokens — real models are nondeterministic).
+- Register the e2e file pattern in `knip.json` (per-workspace `entry`
+  override) or knip flags it unused.