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/support/subagent-mock/src/index.ts

@@ -37,12 +37,14 @@ const DEFAULT_CAPS: SubagentCapabilities = { outputSchema: true, depthLimit: tru
  */
 class MockSubagentProvider implements SubagentProvider {
   readonly capabilities: SubagentCapabilities
+  readonly inheritsParentContext: boolean
 
   constructor(
     readonly name: string,
     private readonly config: Config,
   ) {
     this.capabilities = { ...DEFAULT_CAPS, ...config.capabilities }
+    this.inheritsParentContext = config.inheritsParentContext ?? false
   }
 
   start(request: SubagentStartRequest): SubagentRun {
@@ -88,6 +90,12 @@ export interface Config {
   stopReason?: SubagentStopReason
   /** Which start-time capabilities to advertise (default: all `true`). */
   capabilities?: Partial<SubagentCapabilities>
+  /**
+   * The context contract to declare ({@link SubagentProvider.inheritsParentContext});
+   * default `false` (spawn-like). Set `true` to exercise the fork-shaped tool
+   * wording in consumer tests.
+   */
+  inheritsParentContext?: boolean
   /**
    * Structured value surfaced when a request carries an `outputSchema` and the
    * `outputSchema` capability is on (default: `{ reply }`).
@@ -104,6 +112,7 @@ export const Config: z<Config> = z.object({
     depthLimit: z.boolean(),
     toolFilter: z.boolean(),
   }),
+  inheritsParentContext: z.boolean(),
   structured: z.any(),
 })