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

examples/acp-agent/cordis.yml

@@ -23,9 +23,8 @@
       - deepseek-v4-flash
       - deepseek-v4-pro
 
-# Local bash executor for agent-core's tool-bash schema.
-# FIXME(config-comments): keep this executor note from implying bash is the
-# whole tool set; filesystem, subagent, and todo_write are loaded below.
+# Local bash executor for agent-core's tool-bash schema (one of several tool
+# stacks in this tree: filesystem, subagent, and todo_write load below).
 - id: bash
   name: '@deepseek-ai/dsh-bash-local'
   config:
@@ -39,28 +38,16 @@
   config:
     model: deepseek-v4-flash
     persistenceRoot: !!js process.env.DSH_SNAPSHOT_SESSIONS_ROOT ?? './.sessions'
+    # The persona: identity + behavior only. Tool guidance lives with each tool
+    # plugin (descriptions + prompt sections); {{model}} and {{cwd}} are prompt
+    # variables the agent loop resolves per session (every ACP session carries
+    # the client's cwd, so the persona can state the workspace).
     systemPrompt: |
-      You are a coding assistant driven over the Agent Client Protocol.
+      You are a coding assistant powered by the {{model}} model, driven over
+      the Agent Client Protocol. Your working directory is {{cwd}}.
 
-      Your tools are read/write/edit for file operations, bash (plus
-      bash_output/bash_kill for background tasks), and subagent. Use read to
-      inspect UTF-8 text files, write to create or replace files, and edit for
-      targeted literal replacements. Use bash for shell commands, tests,
-      searches, and operations that are not ordinary file reads or edits. Each
-      bash call runs in a fresh shell — pass workdir instead of cd. Check the
-      [exit code: N] marker; verify your work. Keep answers brief and factual.
-
-      Use the subagent tool to delegate a focused, self-contained subtask to
-      a fresh child agent (it works in its own context and returns only its
-      final result) — give it a complete, standalone instruction. Use
-      subagent_fork instead when the subtask needs THIS conversation's
-      context: the child inherits the log so far.
-
-      For multi-step work, use the todo_write tool to track a task list:
-      send the WHOLE list each call (it replaces the previous one), keep at
-      most one task in_progress (exactly one while work remains), and mark a
-      task completed as soon as it is done. Skip it for trivial single-step
-      tasks.
+      Verify your work by running the code or tests. Keep answers brief and
+      factual.
 
 # The subagent seam + both in-process backends + two model-facing tools, as leaf
 # entries after the app (which provides ctx.agents/ctx.tools). spawn (a fresh