thalamus: add thinking mode toggles (native + tool)

Two independent toggles on the thalamus screen:
- 't' toggles native Qwen <think> tags (adds <think>\n to generation prompt)
- 'T' toggles think tool (Anthropic-style structured reasoning tool)

Both can be enabled simultaneously. Native thinking is on by default.

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

src/agent/tools/mod.rs

@@ -21,6 +21,7 @@ mod write;
 
 // Agent-specific tools
 mod control;
+mod think;
 mod vision;
 
 use std::future::Future;
@@ -190,6 +191,11 @@ pub fn tools() -> Vec<Tool> {
     all
 }
 
+/// The "think" tool for structured reasoning.
+pub fn think_tool() -> Tool {
+    think::tool()
+}
+
 pub async fn all_tool_definitions() -> Vec<String> {
     let mut defs: Vec<String> = tools().iter().map(|t| t.to_json()).collect();
     defs.extend(mcp_client::tool_definitions_json().await);

src/agent/tools/think.rs

@@ -0,0 +1,28 @@
+// tools/think.rs — Structured reasoning tool
+//
+// A tool that does nothing but return its input. Gives the model
+// a structured place to reason before acting — the thinking happens
+// in the tool input, the tool just acknowledges it.
+//
+// Inspired by Anthropic's "think tool" approach:
+// https://www.anthropic.com/engineering/claude-think-tool
+
+use std::sync::Arc;
+
+pub(super) fn tool() -> super::Tool {
+    super::Tool {
+        name: "think",
+        description: "Use this tool to think through a problem step by step before acting. \
+                      Write your reasoning in the 'thought' parameter. The tool returns your \
+                      thought unchanged — it's a scratchpad, not an oracle.",
+        parameters_json: r#"{"type":"object","properties":{"thought":{"type":"string","description":"Your step-by-step reasoning about the current problem"}},"required":["thought"]}"#,
+        handler: Arc::new(|_agent, v| Box::pin(async move {
+            let thought = v.get("thought")
+                .and_then(|v| v.as_str())
+                .unwrap_or("");
+            // Just return the thought — the value is in the model having
+            // a structured place to reason, not in any processing we do.
+            Ok(thought.to_string())
+        })),
+    }
+}