feat(tool-todo): add the model-facing todo_write tool

Add @deepseek-ai/dsh-tool-todo (a new packages/todo/ group): a model-facing
todo_write(todos: [{content, status}]) tool with whole-list-replace semantics.
Each call appends the full list as a todo/write event to the calling agent's
session log; the current list is the most recent such event (last-write-wins).
Single-owner — a non-agent caller is rejected. Beyond the schema's
type/required/enum checks, execute rejects empty/duplicate content and more than
one in_progress task, narrowing the loosely-typed args into a real TodoItem[].

Both UIs render off the existing session/event: the stdio UI prints a glyphed
checklist; the ACP bridge maps the list to a `plan` sessionUpdate (todosToPlan
synthesizes the priority ACP requires; status maps 1:1). Wired into the
coding-agent, acp-agent, and snapshot example configs with a system-prompt nudge.

Tests: unit (schema, validation, append/replace, no-agent rejection, presentCall,
HMR-safety, Loader export-shape guard), full-loop integration through the agent
loop, the ACP todosToPlan mapping + stream-update arm, the stdio render arm, and
a session/load replay that re-emits the plan. New-group TS wiring added to
tsconfig.base/json/build. RFC + a doc-inventory sweep (architecture, packages
README, AGENTS layout, cookbook group list, example READMEs) ship with it.

The todo-plan ACP snapshot scenario is recorded separately (needs an API key).

packages/ui/acp/tests/harness.ts

@@ -20,6 +20,7 @@ import AgentLoop from '@deepseek-ai/dsh-agent-loop'
 import SessionPersistenceJsonl from '@deepseek-ai/dsh-session-persistence-jsonl'
 import { LocalBashExecutor } from '@deepseek-ai/dsh-bash-local'
 import * as ToolBash from '@deepseek-ai/dsh-tool-bash'
+import * as ToolTodo from '@deepseek-ai/dsh-tool-todo'
 import {
   ClientSideConnection,
   ndJsonStream,
@@ -158,6 +159,12 @@ export async function makeBridgeHarness(options: {
    * implementation over a mock in tests").
    */
   withBash?: boolean
+  /**
+   * Plug the REAL `dsh-tool-todo` tool so a test can drive `todo_write` through
+   * the bridge and assert the resulting `plan` sessionUpdate — the shipping
+   * tool + the bridge's own todo/write→plan mapping, not a stand-in.
+   */
+  withTodo?: boolean
 } = { storageDir: '' }): Promise<BridgeHarness> {
   const adapter = new MockAdapter(options.script ?? [])
 
@@ -173,6 +180,9 @@ export async function makeBridgeHarness(options: {
     await ctx.plugin(LocalBashExecutor, { timeoutMs: 10_000 })
     await ctx.plugin(ToolBash)
   }
+  if (options.withTodo) {
+    await ctx.plugin(ToolTodo)
+  }
   ctx.llm.registerAdapter(['mock'], adapter)
 
   // Two identity byte pipes cross-wired into the two ndJsonStreams: bytes the