review: the persona becomes the system-prompt plugin's deployment config

Review round 2 (tianyicui inline comments):

- dsh-system-prompt itself registers the harness:identity (-100) and
  deployment:persona (0) sections — they must survive a swapped loop
  plugin, so they leave dsh-agent-loop; the persona text is the plugin's
  own validated 'persona' config. The model/cwd variables STAY on the
  loop: runtime facts of the agents it drives.
- AgentOptions.systemPrompt is deleted with all its forwarding plumbing:
  the app configs' systemPrompt keys become 'persona' routed through
  dsh-agent-core (schema = z.intersect of the owners'), the ACP bridge
  and tool-subagent stop carrying persona configuration, and subagent
  children now render the deployment persona like every other agent.
- Example personas drop transport/interface trivia (ACP, CLI) — facts
  irrelevant to the model.
- Root CONTEXT.md removed (not idiomatic); its persona definition was
  wrong under the new ownership anyway.
- Docs, READMEs, the prompt-variables RFC, and generated catalogs
  updated; new loop test pins the assemble-waterfall escape valve
  (an emptied assembly sends NO system field).

packages/core/agent-loop/README.md

@@ -28,12 +28,11 @@ interface Config {
   agents: Array<{
     id: string                 // required
     model?: string
-    systemPrompt?: string      // the agent's persona TEMPLATE (may reference {{model}}/{{cwd}})
   }>
 }
 ```
 
-Agents listed in config are auto-created at startup. The plugin also registers the harness-owned prompt pieces on `ctx.systemPrompt`: the `harness:identity` section (order −100 — every agent's prompt opens by stating it is powered by the DeepSeek Harness SDK), the `agent:persona` section (order 0 — `AgentOptions.systemPrompt` renders after it, before all tool guidance), and the built-in `model`/`cwd` prompt variables, resolved per step from the `assemble({ agent })` context.
+Agents listed in config are auto-created at startup. (There is no per-agent persona: the deployment persona is `dsh-system-prompt`'s own `persona` config, shared by every agent in the context.) The plugin registers the built-in `model`/`cwd` prompt variables on `ctx.systemPrompt`, resolved per step from the `assemble({ agent })` context — runtime facts of the agents THIS loop drives, unlike the `harness:identity`/`deployment:persona` sections, which live on `dsh-system-prompt` so they survive a swapped loop plugin.
 
 ### Classes