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/tests/harness.ts

@@ -33,6 +33,11 @@ export const TODO_SYSTEM_PROMPT = 'You are a coding agent. For multi-step work,
 
 /** Options for {@link codingHarness}. */
 export interface CodingHarnessOptions {
+  /**
+   * Deployment persona for the tree (the system-prompt plugin's `persona`
+   * config — per-context, not per-agent). Omitted ⇒ no persona section.
+   */
+  persona?: string
   /** Durable JSONL persistence root (the resume suite needs it; others stay file-free). */
   persistenceRoot?: string
   /**
@@ -47,7 +52,7 @@ export async function codingHarness(workdir: string, options: CodingHarnessOptio
   const ctx = new Context()
   await ctx.plugin(LlmService)
   await ctx.plugin(SessionStore)
-  await ctx.plugin(SystemPrompt)
+  await ctx.plugin(SystemPrompt, { persona: options.persona ?? '' })
   await ctx.plugin(ToolRegistry)
   await ctx.plugin(AgentRegistry)
   await ctx.plugin(AgentLoop, { agents: [] })