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

docs/module-graph.md

@@ -21,6 +21,7 @@ graph TD
   agent --> brand
   agent --> llm
   agent --> session
+  agent --> system-prompt
   compact --> llm
   compact --> session
   fs-local --> fs
@@ -70,6 +71,7 @@ graph TD
   tool-bash --> agent
   tool-bash --> bash
   tool-bash --> llm
+  tool-bash --> system-prompt
   tool-bash --> tools
   tool-fs --> fs
   tool-fs --> llm
@@ -142,7 +144,7 @@ graph TD
 | `session` | `brand`, `llm` |
 | `system-prompt` | `llm` |
 | `web` | `llm` |
-| `agent` | `brand`, `llm`, `session` |
+| `agent` | `brand`, `llm`, `session`, `system-prompt` |
 | `compact` | `llm`, `session` |
 | `fs-local` | `fs` |
 | `fs-policy` | `fs` |
@@ -162,7 +164,7 @@ graph TD
 | `agent-loop` | `agent`, `llm`, `session`, `session-persistence`, `system-prompt`, `tools` |
 | `hooks-codex` | `agent`, `hook-protocol`, `llm`, `session`, `tools` |
 | `subagent` | `agent`, `llm`, `tools` |
-| `tool-bash` | `agent`, `bash`, `llm`, `tools` |
+| `tool-bash` | `agent`, `bash`, `llm`, `system-prompt`, `tools` |
 | `tool-fs` | `fs`, `llm`, `session`, `system-prompt`, `tools` |
 | `tool-todo` | `agent`, `session`, `tools` |
 | `tool-web` | `llm`, `system-prompt`, `tools`, `web` |