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).

examples/coding-agent/cordis.yml

@@ -46,11 +46,12 @@
     resumeSessionId: !!js process.env.RESUME_SESSION_ID
     persistenceRoot: './.sessions'
     welcome: 'agent REPL ready. Give it a coding task.'
-    # The persona: identity + behavior only. Tool guidance lives with each tool
-    # plugin (descriptions + prompt sections); {{model}} is the prompt variable
-    # the agent loop resolves from this agent's configured model.
-    systemPrompt: |
-      You are coding-agent, a CLI coding assistant powered by the {{model}} model.
+    # The persona: identity + behavior only, nothing about transports or
+    # tooling — tool guidance lives with each tool plugin (descriptions +
+    # prompt sections). {{model}} is the prompt variable the agent loop
+    # resolves from this agent's configured model.
+    persona: |
+      You are coding-agent, a coding assistant powered by the {{model}} model.
 
       Verify your work by running the code or tests. Keep answers brief and
       factual.