feat(system-prompt): prompt variables, persona-as-section, tool-guidance ownership

One principle: every fact in the assembled prompt has exactly one owner.

- dsh-system-prompt: merge-extensible AssembleContext on assemble();
  a variable(name, provider) registry; {{name}} interpolation in
  renderPrompt, strict (unknown/valueless/malformed references throw);
  duplicate section and variable names rejected; assembly carries
  resolved section text + variables through the assemble waterfall.
- dsh-agent declares AssembleContext.agent; dsh-agent-loop registers
  the agent:persona section (order 0 - identity renders before tool
  guidance) and the model/cwd variables, and drops its string join:
  renderPrompt(assembly) IS the full prompt.
- Tool guidance moves to its owners: descriptions carry per-tool
  semantics; sections only cross-call habits (tool:bash exit-code
  habit at order 105; read's not-shell nudge). todo/subagent need no
  section - their descriptions already carry the contract.
- SubagentProvider.inheritsParentContext (spawn/acp false, fork true);
  dsh-tool-subagent derives truthful per-provider wording and resolves
  the provider at load (backend must be listed first).
- Example personas shrink to identity + behavior with {{model}} (and
  {{cwd}} in the ACP tree); the welcome banner stops enumerating tools.

RFC: docs/rfc/implemented/architecture/2026-07-05-prompt-variables-and-tool-guidance-ownership.md

packages/ui/stdio-agent/README.md

@@ -24,7 +24,7 @@ The leaf `cordis.yml` supplies only the **swappable backends** — an LLM adapte
 | Key | Default | Routed to |
 |---|---|---|
 | `model` | (required) | the pre-created `main` agent's model |
-| `systemPrompt` | (required) | the `main` agent's system prompt |
+| `systemPrompt` | (required) | the `main` agent's persona template (may reference `{{model}}`) |
 | `persistenceRoot` | `./.sessions` | the JSONL backend's root directory |
 | `welcome` | `ready.` | the stdin-chat banner |
 | `resumeSessionId` | — | resume a persisted session id instead of starting fresh (sourced from an env var in the leaf) |
@@ -54,7 +54,7 @@ The leaf `cordis.yml` supplies only the **swappable backends** — an LLM adapte
   name: '@deepseek-ai/dsh-stdio-agent'
   config:
     model: deepseek-v4-flash
-    systemPrompt: 'You are a CLI coding assistant. Your only tools are bash…'
+    systemPrompt: 'You are a CLI coding assistant powered by the {{model}} model.'
 ```
 
 Swap `llm-deepseek` for a `mock-llm` leaf plugin and you have the echo demo — "swap the backend, keep the app".