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/load.spec.ts

@@ -94,6 +94,44 @@ describe('acp bridge — session/load replay', () => {
     expect(content[0]?.content.text).toBe('```console\nhello\n```')
   })
 
+  it('replays a persisted todo/write as a plan sessionUpdate on load', async () => {
+    // A turn whose model called todo_write persists a todo/write event. A fresh
+    // bridge loading the session must re-emit the ACP `plan` update from the log
+    // (the load replay runs every event through streamSessionEventUpdate), so an
+    // editor reopening the session sees the current plan.
+    live = await makeBridgeHarness({
+      storageDir,
+      withTodo: true,
+      script: [
+        toolCallResponse('c1', 'todo_write', {
+          todos: [
+            { content: 'first step', status: 'in_progress' },
+            { content: 'second step', status: 'pending' },
+          ],
+        }),
+        textResponse('planned'),
+      ],
+    })
+    await live.client.initialize({ protocolVersion: PROTOCOL_VERSION, clientCapabilities: {} })
+    const { sessionId } = await live.client.newSession({ cwd: process.cwd(), mcpServers: [] })
+    await live.client.prompt({ sessionId, prompt: [{ type: 'text', text: 'plan it' }] })
+    await live.dispose()
+    live = undefined
+
+    loader = await makeBridgeHarness({ storageDir, withTodo: true, script: [] })
+    await loader.client.initialize({ protocolVersion: PROTOCOL_VERSION, clientCapabilities: {} })
+    await loader.client.loadSession({ sessionId, cwd: process.cwd(), mcpServers: [] })
+
+    const plan = loader.updates.find(u => u.sessionUpdate === 'plan')
+    expect(plan).toEqual({
+      sessionUpdate: 'plan',
+      entries: [
+        { content: 'first step', priority: 'medium', status: 'in_progress' },
+        { content: 'second step', priority: 'medium', status: 'pending' },
+      ],
+    })
+  })
+
   it('replays a persisted bash call as a TERMINAL card when the loader advertises the capability', async () => {
     // The presentation is resolved at replay time, so a loader that advertised
     // _meta.terminal_output must reconstruct the terminal card (content + _meta)