fix(tools): address Codex review of arg validation (PR 1)

- enum membership now checked uniformly for all SchemaTypes, mirroring the
  converter which emits `enum` regardless of type (was string-only)
- checkValue switch ends in assertNever per the closed-union convention
- sync the adding-a-tool cookbook to the validate-for-you behavior
- soften ADR 0011's property-test claim (RFC 001 not yet landed)

docs/cookbook/adding-a-tool.md

@@ -32,7 +32,7 @@ Registration is effect-based: disposing the plugin fiber unregisters the tool (w
 
 ## Rules of the execute() contract
 
-- **Validate args at runtime.** `defineTool`'s `InferArgs` typing is compile-time only; at runtime `arguments` is whatever JSON the model emitted. Check every field; throw a descriptive Error for bad input.
+- **Args are validated for you.** `defineTool` validates the model-generated `arguments` against the `SchemaSpec` before `execute` runs (type, required keys, enum membership, nested objects/arrays — ADR 0011), so inside `execute` the args already match `InferArgs`. You still hand-check value constraints the DSL can't express (non-empty strings, positive numbers, cross-field rules); throw a descriptive Error for those. Raw JSON-Schema tools registered directly (MCP) are NOT validated by the harness — they validate their own input.
 - **Throwing means isError.** The registry catches anything `execute()` throws and returns `{isError: true}` to the model. Use that for infrastructure failures (bad input, spawn errors, aborts) — but REPORT domain failures in the result text instead (e.g. tool-bash returns `[exit code: 9]` with `isError: false`: the model decides what a failing command means).
 - **Honor `exec.signal`.** Cancel in-flight work when it fires.
 - **Use `exec.agent` for async notifications.** `agent.inject(content, {source: {kind: 'plugin', plugin: '<name>'}})` appends durable context the NEXT model request sees — it is not a wake-up (an idle agent stays idle). Guard against disposed agents (try/catch).