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/ui/acp-agent/tests/acp-agent.spec.ts

@@ -24,7 +24,7 @@ async function mount(config: acpAgent.Config): Promise<Context> {
 
 describe('dsh-acp-agent composition', () => {
   it('brings up the spine + persistence + the ACP bridge', async () => {
-    const ctx = await mount({ model: 'mock', systemPrompt: 'hi', persistenceRoot: '/tmp/dsh-acp-agent-test' })
+    const ctx = await mount({ model: 'mock', persona: 'hi', persistenceRoot: '/tmp/dsh-acp-agent-test' })
     expect(ctx.get('agents')).toBeDefined()
     expect(ctx.get('sessions')).toBeDefined()
     expect(ctx.get('sessionPersistence')).toBeDefined()
@@ -40,7 +40,8 @@ describe('dsh-acp-agent composition', () => {
     // `ctx.plugin`, which validates+defaults the config first) with no
     // persistenceRoot, so the runtime fallback is the one that fires.
     const ctx = new Context()
-    acpAgent.apply(ctx, { model: 'mock', systemPrompt: 'hi' })
+    // No persona: covers the omitted-persona forwarding branch too.
+    acpAgent.apply(ctx, { model: 'mock' })
     await new Promise(resolve => setTimeout(resolve, 50))
     expect(ctx.get('sessionPersistence')).toBeDefined()
     await ctx.fiber.dispose()