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/coding-task.e2e.ts

@@ -53,11 +53,8 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('coding task: fix a failing test
     const before = spawnSync('node', ['add.test.js'], { cwd: workdir })
     expect(before.status).not.toBe(0)
 
-    ctx = await codingHarness(workdir)
-    const agent = ctx.agentLoop.create(AgentId('e2e-task'), {
-      model: 'deepseek-v4-flash',
-      systemPrompt: SYSTEM_PROMPT,
-    })
+    ctx = await codingHarness(workdir, { persona: SYSTEM_PROMPT })
+    const agent = ctx.agentLoop.create(AgentId('e2e-task'), { model: 'deepseek-v4-flash' })
 
     agent.send([{
       type: 'text',