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/subagent/tool-subagent/tests/tool-subagent.spec.ts

@@ -149,10 +149,10 @@ describe('dsh-tool-subagent', () => {
         }
       },
     })
-    await ctx.plugin(tool, { provider: 'capture', agentOptions: { model: 'child-model', systemPrompt: 'be terse' } })
+    await ctx.plugin(tool, { provider: 'capture', agentOptions: { model: 'child-model' } })
 
     await callSubagent(ctx, { description: 'd', prompt: 'p' })
-    expect(seen?.agentOptions).toEqual({ model: 'child-model', systemPrompt: 'be terse' })
+    expect(seen?.agentOptions).toEqual({ model: 'child-model' })
   })
 
   it('defaults toolName and omits agentOptions when apply() is called directly (schema bypass)', async () => {