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/stream-update.spec.ts

@@ -3,7 +3,7 @@ import { CallId } from '@deepseek-ai/dsh-llm'
 import { SessionId, type SessionEvent } from '@deepseek-ai/dsh-session'
 import type { SessionNotification } from '@agentclientprotocol/sdk'
 import type { ToolDefinition, ToolRegistry } from '@deepseek-ai/dsh-tools'
-import { streamSessionEventUpdate, agentOptions, ToolPresenter } from '../src/index.ts'
+import { streamSessionEventUpdate, agentOptions, todosToPlan, ToolPresenter } from '../src/index.ts'
 
 /** Collect the updates a single event produces (no presenter → generic fallback). */
 function updatesFor(event: SessionEvent): SessionNotification['update'][] {
@@ -118,6 +118,43 @@ describe('streamSessionEventUpdate', () => {
     expect(updatesFor(evt('turn/end', { turn: 1, reason: { kind: 'completed' } }))).toEqual([])
     expect(updatesFor(evt('step/start', { turn: 1, step: 1 }))).toEqual([])
   })
+
+  it('maps todo/write to a plan sessionUpdate with priority synthesized as medium', () => {
+    expect(updatesFor(evt('todo/write', {
+      todos: [
+        { content: 'plan the work', status: 'in_progress' },
+        { content: 'write the code', status: 'pending' },
+        { content: 'run the tests', status: 'completed' },
+      ],
+    }))).toEqual([{
+      sessionUpdate: 'plan',
+      entries: [
+        { content: 'plan the work', priority: 'medium', status: 'in_progress' },
+        { content: 'write the code', priority: 'medium', status: 'pending' },
+        { content: 'run the tests', priority: 'medium', status: 'completed' },
+      ],
+    }])
+  })
+
+  it('maps an empty todo list to a plan with no entries', () => {
+    expect(updatesFor(evt('todo/write', { todos: [] }))).toEqual([{ sessionUpdate: 'plan', entries: [] }])
+  })
+})
+
+describe('todosToPlan', () => {
+  it('maps status 1:1 and stamps every entry priority medium', () => {
+    expect(todosToPlan([
+      { content: 'a', status: 'pending' },
+      { content: 'b', status: 'in_progress' },
+      { content: 'c', status: 'completed' },
+    ])).toEqual({
+      entries: [
+        { content: 'a', priority: 'medium', status: 'pending' },
+        { content: 'b', priority: 'medium', status: 'in_progress' },
+        { content: 'c', priority: 'medium', status: 'completed' },
+      ],
+    })
+  })
 })
 
 describe('ToolPresenter (tool-owned presentation via the tool registry)', () => {