refactor(mode): move exit-tool visibility onto the registry restriction layer; register /mode; drop unconsumed surfaces

The master merge brought the tool registry's per-scope restriction layer
(tools.restrict), which makes dsh-mode's prepend assemble filter a duplicate
enforcement shape: it re-implemented the registry's SDK-section rendering
(renderToolsSdk + the RUN_CODE_NAME exclusion) and hid the exit tool from
prompts only — dispatch stayed open and the execute-time re-check was the
real gate. The service now reconciles a per-agent deny restriction on
agent.ctx at agent/created and at every boundary flush, so wire schemas, the
Code Mode tools:sdk section, AND dispatch resolve exit_plan_mode through the
one registry view (a default-mode call answers UNKNOWN_TOOL, byte-identical
to a no-dsh-mode deployment). The execute-time folded-mode re-check stays as
defense in depth for a direct foreign mode/set append no boundary has
reconciled yet.

Two zero-consumer surfaces removed per the pre-release stance:
- AgentOptions.mode creation seeding (declaration merge + agent/created
  listener); a caller selects through set() before the first turn, and the
  deferred subagent inheritance returns together with its consumer.
- The dropped-definition boundary notice (droppedNoticed + narration):
  custom mode definitions have no production consumer, so nothing can be
  dropped; fold-to-default degradation is unchanged.

The stdio removal had left plan mode ACP-only while docs still claimed a
/mode command. dsh-mode now registers /mode on the plugin-owned command
registry through an optional ctx.inject(['commands']) child (type-only peer
edge on dsh-commands), so the TUI and the ACP slash-command surface both
gain it; examples/tui-agent composes dsh-mode. ACP/TUI expected outputs
refreshed keyless for the available_commands_update delta.

Docs updated in place (mode READMEs, the plan-mode Agent Note's realization
sections); catalogs and graphs regenerated.

examples/tui-agent/composition.md

@@ -47,6 +47,8 @@ flowchart LR
   cfg --> plugin_tui_tool_ralph
   plugin_tui_tool_todo["tool-todo<br/>@deepseek-ai/dsh-tool-todo"]
   cfg --> plugin_tui_tool_todo
+  plugin_tui_mode["mode<br/>@deepseek-ai/dsh-mode"]
+  cfg --> plugin_tui_mode
   plugin_tui_fs_local["fs-local<br/>@deepseek-ai/dsh-fs-local"]
   cfg --> plugin_tui_fs_local
   plugin_tui_fs_policy["fs-policy<br/>@deepseek-ai/dsh-fs-policy"]
@@ -81,6 +83,7 @@ flowchart LR
 | `tool-workflow` | `@deepseek-ai/dsh-tool-workflow` |
 | `tool-ralph` | `@deepseek-ai/dsh-tool-ralph` |
 | `tool-todo` | `@deepseek-ai/dsh-tool-todo` |
+| `mode` | `@deepseek-ai/dsh-mode` |
 | `fs-local` | `@deepseek-ai/dsh-fs-local` |
 | `fs-policy` | `@deepseek-ai/dsh-fs-policy` |
 | `tool-fs` | `@deepseek-ai/dsh-tool-fs` |