feat(helexa-acp): wire ACP agent loop for text-only conversations

Stage 2 lands the agent loop on top of the Stage 1 scaffold: session
state with per-session cancellation, a system-prompt builder honouring
HELEXA_ACP_SYSTEM_PROMPT_PATH / system_prompt_path TOML, and handlers
for initialize / session/new / session/prompt / session/cancel that
stream provider output back as session/update notifications. Verified
end-to-end against cortex from Zed.

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

crates/helexa-acp/src/config.rs

@@ -134,7 +134,10 @@ impl EndpointConfig {
         join_segments(&self.base_url, &["chat", "completions"])
     }
 
-    /// `{base_url}/models`.
+    /// `{base_url}/models`. Called from `Provider::list_models`, which
+    /// Stage 4 wires into the model-picker dropdown; until then it's
+    /// reachable code with no in-tree callers.
+    #[allow(dead_code)]
     pub fn models_url(&self) -> Url {
         join_segments(&self.base_url, &["models"])
     }