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/README.md

@@ -2,7 +2,9 @@
 
 The model-facing bash tools — `bash`, `bash_output`, `bash_kill` — registered over the `ctx.bash` executor seam (`@deepseek-ai/dsh-bash`). Pure schema + text shaping; every process concern lives behind the seam, so sandboxed or remote executor implementations swap in without changing what the model sees.
 
-Requires a loaded executor implementation (e.g. `@deepseek-ai/dsh-bash-local`); the plugin stays pending until `ctx.bash` exists (`inject: ['tools', 'bash']`).
+Requires a loaded executor implementation (e.g. `@deepseek-ai/dsh-bash-local`); the plugin stays pending until `ctx.bash` exists (`inject: ['tools', 'bash', 'systemPrompt']`).
+
+The plugin also contributes the `tool:bash` prompt section (order 105) — the cross-call habit the per-tool descriptions cannot carry: check the `[exit code: N]` marker on every result and investigate failures before moving on.
 
 ## Tools
 

packages/bash/tool-bash/package.json

@@ -25,6 +25,7 @@
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-bash": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
+    "@deepseek-ai/dsh-system-prompt": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
   },

packages/bash/tool-bash/src/index.ts

@@ -43,11 +43,12 @@ import { isAbsolute, resolve as resolvePath } from 'node:path'
 import { defineTool } from '@deepseek-ai/dsh-tools'
 import type { GenericCallView, TerminalCallView, ToolResult, ToolResultView } from '@deepseek-ai/dsh-tools'
 import type { Agent } from '@deepseek-ai/dsh-agent'
+import type {} from '@deepseek-ai/dsh-system-prompt'
 import { BashTaskId, OwnerToken } from '@deepseek-ai/dsh-bash'
 import type { BashRunResult, BashTask, CollectedOutput } from '@deepseek-ai/dsh-bash'
 
 export const name = 'tool-bash'
-export const inject = ['tools', 'bash']
+export const inject = ['tools', 'bash', 'systemPrompt']
 
 /**
  * Validate the constraints the SchemaSpec can't express. `defineTool` now
@@ -285,6 +286,15 @@ function statusLine(task: BashTask): string {
 }
 
 export function apply(ctx: Context): void {
+  // The bash tools' cross-call HABIT, which the per-tool descriptions cannot
+  // carry (they describe one call each): the exit-code marker is only useful
+  // if the model actually checks it every time.
+  ctx.systemPrompt.section({
+    name: 'tool:bash',
+    order: 105,
+    text: 'Check the [exit code: N] marker on every bash result; investigate failures before moving on.',
+  })
+
   /**
    * The caller's owner TOKEN — the owning agent's `session.header.id`, or
    * `undefined` for a non-agent caller. Read `session.header.id` (NOT

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 () => {