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/README.md

@@ -14,6 +14,7 @@ Load it as a plugin (functional shape: `name`/`inject`/`Config`/`apply`, no defa
 | `reply` | `mock subagent reply` | The scripted child's final answer text. |
 | `stopReason` | `completed` | The stop reason `result` settles with. |
 | `capabilities` | all `true` | Which start-time capabilities (`outputSchema`/`depthLimit`/`toolFilter`) the provider advertises. |
+| `inheritsParentContext` | `false` | The context contract to declare; `true` exercises the fork-shaped tool wording in consumer tests. |
 | `structured` | `{ reply }` | Structured value surfaced when a request carries an `outputSchema` and the capability is on. |
 
 A `cancel()` issued before `result` settles flips the stop reason to `aborted`, so the cancellation path is observable.