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/core/agent-loop/src/loop.ts

@@ -152,7 +152,8 @@ export interface LoopHandle {
  *     every prompt blocked → 'turn/end'(rejected), 0 steps
  *     STEP loop:
  *       drain steering → session('steering/message')  ⟵ catches late steering
- *       assembly = ctx.systemPrompt.assemble()        ⟵ waterfall system-prompt/assemble
+ *       assembly = ctx.systemPrompt.assemble({agent}) ⟵ waterfall system-prompt/assemble; renderPrompt
+ *                                                        (persona section + {{variables}}) IS the full prompt
  *       await ctx.serial('agent/pre-step')            ⟵ surface mutation (compaction) OUTSIDE the step
  *       session('step/start')                         ⟵ durable step boundary (no agent/* mirror)
  *       req = {model, system, tools, messages: session.deriveMessages(), signal}
@@ -434,11 +435,11 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       // because the pre-step seam needs it: compaction measures token pressure
       // against the system prompt (it counts toward the budget). runStep reuses
       // this same assembly for the request, so the prompt is assembled once per
-      // step.
-      const assembly = await ctx.systemPrompt.assemble()
-      const fullSystemPrompt = [renderPrompt(assembly), agent.options.systemPrompt ?? '']
-        .filter(text => text.length > 0)
-        .join('\n\n')
+      // step. renderPrompt IS the full prompt — the persona is the order-0
+      // section (registered by the AgentLoop plugin) and `{{variable}}`
+      // interpolation happens in the render, so there is no separate join.
+      const assembly = await ctx.systemPrompt.assemble({ agent })
+      const fullSystemPrompt = renderPrompt(assembly)
 
       // Interruption landing after assembly: dispose() or cancel() in a
       // turn-start listener (or a listener whose promise resolved before the