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/channels/email_channel.rs

@@ -154,7 +154,14 @@ impl EmailChannel {
                 _ => {}
             }
         }
-        result.split_whitespace().collect::<Vec<_>>().join(" ")
+        let mut normalized = String::with_capacity(result.len());
+        for word in result.split_whitespace() {
+            if !normalized.is_empty() {
+                normalized.push(' ');
+            }
+            normalized.push_str(word);
+        }
+        normalized
     }
 
     /// Extract the sender address from a parsed email