Add ConversationEntry::Thinking — 0 tokens, not sent to API

Thinking/reasoning content is now a first-class entry type:
- Serialized as {"thinking": "..."} in conversation log
- 0 tokens for budgeting (doesn't count against context window)
- Filtered from assemble_api_messages (not sent back to model)
- Displayed in UI with "thinking: ..." label and expandable content

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

src/agent/mod.rs

@@ -305,7 +305,7 @@ impl Agent {
         }
         // Conversation entries
         msgs.extend(self.context.conversation.entries().iter()
-            .filter(|e| !e.entry.is_log())
+            .filter(|e| !e.entry.is_log() && !e.entry.is_thinking())
             .map(|e| e.entry.api_message().clone()));
         msgs
     }
@@ -324,7 +324,7 @@ impl Agent {
                 eprintln!("warning: failed to log entry: {:#}", e);
             }
         }
-        let tokens = if entry.is_log() { 0 } else {
+        let tokens = if entry.is_log() || entry.is_thinking() { 0 } else {
             context::msg_token_count(&self.tokenizer, entry.api_message())
         };
         self.context.conversation.push(ContextEntry {