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/bash/tool-bash/tests/tools.spec.ts

@@ -261,6 +261,14 @@ describe('bash tool', () => {
     })
   })
 
+  it('contributes the exit-code habit as its prompt section (guidance the descriptions cannot carry)', async () => {
+    const ctx = await setup()
+    const assembly = await ctx.systemPrompt.assemble()
+    const section = assembly.sections.find(s => s.name === 'tool:bash')
+    expect(section?.order).toBe(105)
+    expect(section?.text).toContain('[exit code: N]')
+  })
+
   it('unregisters everything when the plugin fiber is disposed (HMR safety)', async () => {
     const ctx = new Context()
     await ctx.plugin(SystemPrompt)
@@ -268,8 +276,10 @@ describe('bash tool', () => {
     await ctx.plugin(LocalBashExecutor, {})
     const fiber = await ctx.plugin(ToolBash)
     expect(ctx.tools.schemas()).toHaveLength(3)
+    expect((await ctx.systemPrompt.assemble()).sections.map(s => s.name)).toEqual(['tool:bash'])
     await fiber.dispose()
     expect(ctx.tools.schemas()).toHaveLength(0)
+    expect((await ctx.systemPrompt.assemble()).sections).toHaveLength(0)
   })
 
   it('tools depend on the executor: no registration without ctx.bash', async () => {