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/fs/tool-fs/tests/tools.spec.ts

@@ -133,10 +133,11 @@ describe('registration', () => {
     // withdraw both, not just the schemas.
     expect(ctx.tools.schemas()).toHaveLength(3)
     const sectionNames = (a: { sections: { name: string }[] }) => a.sections.map(s => s.name).sort()
-    expect(sectionNames(await ctx.systemPrompt.assemble())).toEqual(['tool:edit', 'tool:read', 'tool:write'])
+    expect(sectionNames(await ctx.systemPrompt.assemble())).toEqual(['deployment:persona', 'harness:identity', 'tool:edit', 'tool:read', 'tool:write'])
     await fiber.dispose()
     expect(ctx.tools.schemas()).toHaveLength(0)
-    expect((await ctx.systemPrompt.assemble()).sections).toHaveLength(0)
+    // Only the system-prompt plugin's own built-in sections remain.
+    expect(sectionNames(await ctx.systemPrompt.assemble())).toEqual(['deployment:persona', 'harness:identity'])
   })
 })