perf: eliminate unnecessary heap allocations across agent loop, memory, and channels

- Replace clone()+clear() with std::mem::take() in chunker (items 1, 6)
- Add Vec::with_capacity() hints in chunker split functions (item 2)
- Replace collect::<Vec<_>>().join() with direct iteration in IRC and
  email channels (item 3)
- Share heading strings via Rc<str> instead of cloning per chunk (item 5)
- Use borrowed references in provider tool spec types to avoid cloning
  name/description/parameters per tool per request (item 7)

Closes #712

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/providers/openai.rs

@@ -54,12 +54,12 @@ impl ResponseMessage {
 }
 
 #[derive(Debug, Serialize)]
-struct NativeChatRequest {
+struct NativeChatRequest<'a> {
     model: String,
     messages: Vec<NativeMessage>,
     temperature: f64,
     #[serde(skip_serializing_if = "Option::is_none")]
-    tools: Option<Vec<NativeToolSpec>>,
+    tools: Option<Vec<NativeToolSpec<'a>>>,
     #[serde(skip_serializing_if = "Option::is_none")]
     tool_choice: Option<String>,
 }
@@ -75,18 +75,18 @@ struct NativeMessage {
     tool_calls: Option<Vec<NativeToolCall>>,
 }
 
-#[derive(Debug, Serialize, Deserialize)]
-struct NativeToolSpec {
+#[derive(Debug, Serialize)]
+struct NativeToolSpec<'a> {
     #[serde(rename = "type")]
-    kind: String,
-    function: NativeToolFunctionSpec,
+    kind: &'static str,
+    function: NativeToolFunctionSpec<'a>,
 }
 
-#[derive(Debug, Serialize, Deserialize)]
-struct NativeToolFunctionSpec {
-    name: String,
-    description: String,
-    parameters: serde_json::Value,
+#[derive(Debug, Serialize)]
+struct NativeToolFunctionSpec<'a> {
+    name: &'a str,
+    description: &'a str,
+    parameters: &'a serde_json::Value,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -150,16 +150,16 @@ impl OpenAiProvider {
         }
     }
 
-    fn convert_tools(tools: Option<&[ToolSpec]>) -> Option<Vec<NativeToolSpec>> {
+    fn convert_tools<'a>(tools: Option<&'a [ToolSpec]>) -> Option<Vec<NativeToolSpec<'a>>> {
         tools.map(|items| {
             items
                 .iter()
                 .map(|tool| NativeToolSpec {
-                    kind: "function".to_string(),
+                    kind: "function",
                     function: NativeToolFunctionSpec {
-                        name: tool.name.clone(),
-                        description: tool.description.clone(),
-                        parameters: tool.parameters.clone(),
+                        name: &tool.name,
+                        description: &tool.description,
+                        parameters: &tool.parameters,
                     },
                 })
                 .collect()