Remove dead code: scan_pid_files, backend_label, entries_mut, post_json

All confirmed unused anywhere in src/ or channels/.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -187,14 +187,6 @@ impl ApiClient {
     pub fn base_url(&self) -> &str { &self.base_url }
     pub fn api_key(&self) -> &str { &self.api_key }
 
-    /// Return a label for the active backend, used in startup info.
-    pub fn backend_label(&self) -> &str {
-        if self.base_url.contains("openrouter") {
-            "openrouter"
-        } else {
-            "openai-compat"
-        }
-    }
 }
 
 /// Send an HTTP request and check for errors. Shared by both backends.