feat(helexa-acp): inject and parse Qwen3 Hermes tool format

The OpenAI `tools` API field isn't load-bearing in this stack —
neuron's chat template renders only message.content, so tool
definitions sent that way never reach the model. Move both sides
of the tool conversation into the Qwen3 Hermes wire format the
model is actually trained on:

- Append a `# Tools` block to the system prompt describing every
  available function (qwen3::render_tool_block).
- Parse `<tool_call>{json}</tool_call>` markers out of the streamed
  content via a chunk-boundary-safe state machine (qwen3::ToolCallParser),
  surfacing them as the existing CompletionEvent::ToolCall* events
  so the agent loop doesn't change.
- Re-serialise assistant turns that called tools with inline
  `<tool_call>` blocks and tool results as user turns wrapped in
  `<tool_response>` (qwen3::render_assistant_with_tool_calls,
  render_tool_response).

Verified against cortex+Qwen3.6-27B: the model produces a
well-formed `<tool_call>{"name":"list_dir","arguments":{"path":"/tmp"}}</tool_call>`
in response to a Hermes-formatted prompt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/helexa-acp/src/prompt.rs

@@ -1,53 +1,59 @@
 //! System prompt assembly.
 //!
-//! The built-in prompt tells the model the working directory and
-//! enumerates the tools it actually has — without this, models trained
-//! to "be safe when you don't know your environment" tend to refuse
-//! tool use and ask the user to paste content instead. Override with
-//! `HELEXA_ACP_SYSTEM_PROMPT_PATH` (env) or `system_prompt_path`
-//! (TOML); the literal token `{cwd}` in a user-supplied file is
-//! substituted with the session's working directory.
+//! The system message has two parts:
+//!
+//! 1. A short human-readable preamble (working directory, style
+//!    instructions). Either the built-in [`DEFAULT_PROMPT`] or a
+//!    user-supplied file at `HELEXA_ACP_SYSTEM_PROMPT_PATH` /
+//!    `system_prompt_path`. `{cwd}` is substituted in both.
+//! 2. A `# Tools` block in Qwen3 Hermes format (see [`crate::qwen3`])
+//!    describing the available functions. This is what makes the
+//!    model actually call them — neuron/cortex don't honour the
+//!    OpenAI `tools` API field, so the tool list has to live in the
+//!    prompt itself.
 
 use anyhow::Context;
 use std::path::Path;
 
+use crate::provider::ToolSpec;
+use crate::qwen3;
+
 const DEFAULT_PROMPT: &str = "\
 You are helexa-acp, a coding assistant working inside an editor.
 
 Working directory: {cwd}
 
-You have the following tools. Call them whenever the user's request
-involves looking at or modifying files, or running commands — do not
-ask the user to paste file contents you could read yourself.
-
-- read_file(path, line?, limit?) — Read a text file's contents.
-- write_file(path, content) — Create or overwrite a file.
-- edit_file(path, old_text, new_text) — Replace one unique substring
-  in a file. Fails if old_text is not unique; call multiple times for
-  multiple edits.
-- list_dir(path) — List a directory's entries.
-- bash(command, cwd?) — Run a shell command via `sh -c`. Returns
-  combined stdout+stderr and the exit status.
-
-All file paths must be absolute. Writes and shell commands may
-prompt the user for permission depending on the session mode.
+Use the tools described below whenever the user's request involves
+looking at or modifying files, or running commands. Do not ask the
+user to paste file contents you could read yourself. All file paths
+must be absolute. Writes and shell commands may prompt the user for
+permission depending on the session mode.
 
 Be concise; the user is reading your output in an editor pane.";
 
 /// Build the system prompt for a session.
 ///
-/// `cwd` is the session's working directory (substituted for `{cwd}`
-/// in both the default prompt and any user-supplied template).
-/// `override_path` is the user's `system_prompt_path` (TOML) or
-/// `HELEXA_ACP_SYSTEM_PROMPT_PATH` (env) value, already resolved by
-/// [`crate::config::Config`].
-pub fn build_system_prompt(cwd: &Path, override_path: Option<&Path>) -> anyhow::Result<String> {
+/// - `cwd`: session working directory (substituted for `{cwd}` in
+///   the preamble — both the default and any user-supplied template).
+/// - `override_path`: path to a user-supplied template, already
+///   resolved by [`crate::config::Config`]. The `# Tools` block is
+///   appended *after* the user's template so a custom preamble
+///   still gets the tool descriptions the model needs.
+/// - `tools`: the tools to advertise. Empty list → no `# Tools`
+///   block is appended at all.
+pub fn build_system_prompt(
+    cwd: &Path,
+    override_path: Option<&Path>,
+    tools: &[ToolSpec],
+) -> anyhow::Result<String> {
     let template = match override_path {
         Some(path) => std::fs::read_to_string(path)
             .with_context(|| format!("read system prompt from {}", path.display()))?,
         None => DEFAULT_PROMPT.to_string(),
     };
-    Ok(template.replace("{cwd}", &cwd.display().to_string()))
+    let mut prompt = template.replace("{cwd}", &cwd.display().to_string());
+    prompt.push_str(&qwen3::render_tool_block(tools));
+    Ok(prompt)
 }
 
 #[cfg(test)]
@@ -57,7 +63,7 @@ mod tests {
 
     #[test]
     fn default_prompt_substitutes_cwd() {
-        let prompt = build_system_prompt(Path::new("/home/me/proj"), None).unwrap();
+        let prompt = build_system_prompt(Path::new("/home/me/proj"), None, &[]).unwrap();
         assert!(
             prompt.contains("/home/me/proj"),
             "cwd not interpolated: {prompt}"
@@ -67,6 +73,22 @@ mod tests {
             !prompt.contains("{cwd}"),
             "left-over placeholder in default prompt"
         );
+        // With no tools, the # Tools block is absent.
+        assert!(!prompt.contains("# Tools"));
+    }
+
+    #[test]
+    fn tools_are_appended_in_hermes_format() {
+        let spec = ToolSpec {
+            name: "read_file".into(),
+            description: "Read a file.".into(),
+            parameters: serde_json::json!({"type":"object","properties":{}, "required":[]}),
+        };
+        let prompt = build_system_prompt(Path::new("/x"), None, &[spec]).unwrap();
+        assert!(prompt.contains("# Tools"));
+        assert!(prompt.contains("<tools>"));
+        assert!(prompt.contains("\"name\":\"read_file\""));
+        assert!(prompt.contains("<tool_call>"));
     }
 
     #[test]
@@ -78,8 +100,8 @@ mod tests {
         let path = tmp.path().to_path_buf();
         drop(tmp);
 
-        let prompt =
-            build_system_prompt(Path::new("/etc"), Some(path.as_path())).expect("read override");
+        let prompt = build_system_prompt(Path::new("/etc"), Some(path.as_path()), &[])
+            .expect("read override");
         assert_eq!(prompt, "custom prompt for /etc only");
 
         let _ = std::fs::remove_file(&path);
@@ -90,6 +112,7 @@ mod tests {
         let err = build_system_prompt(
             Path::new("/tmp"),
             Some(Path::new("/definitely/not/a/real/path")),
+            &[],
         )
         .unwrap_err();
         assert!(format!("{err:#}").contains("read system prompt"));