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).

AGENTS.md

@@ -73,6 +73,10 @@ packages/    Harness packages, grouped by role at packages/<group>/<pkg>/.
     bash/           abstract bash executor seam (ctx.bash) — interface only
     bash-local/     local-subprocess BashExecutor implementation
     tool-bash/      model-facing bash/bash_output/bash_kill tool schemas
+  todo/           todo/planning capability family
+    tool-todo/      model-facing todo_write tool: writes the whole task list to
+                    the session log (todo/write), rendered as a stdio checklist /
+                    ACP plan
   session-persistence/   persistence capability family
     session-persistence/         durable persistence seam + write coordinator
     session-persistence-jsonl/    JSONL-sidecar backend