tools: promote reasoning_think to builtin

Zero-context-cost scratchpad tool. The thought is recorded in history
but returns empty — no output tokens wasted. Registered in ListTools
schema and builtins map.

tools/builtins.go

@@ -8,6 +8,7 @@ import "ollie/toolsrv"
 func Builtins() map[string]toolsrv.Handler {
 	return map[string]toolsrv.Handler{
 		"shell":           Shell,
+		"reasoning_think": Think,
 		"tool_list":       ToolList,
 		"tool_load":       ToolLoad,
 		"tool_active":     ToolActive,

tools/reasoning.go

@@ -0,0 +1,25 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"ollie/toolsrv"
+)
+
+// Think is a scratchpad for externalizing reasoning. The thought is recorded
+// in conversation history but not shown to the user. Takes up practically
+// zero context space — the value is in writing it.
+func Think(ctx context.Context, srv *toolsrv.Server, args json.RawMessage) (string, error) {
+	var a struct {
+		Thought string `json:"thought"`
+	}
+	if err := json.Unmarshal(args, &a); err != nil {
+		return "", fmt.Errorf("reasoning_think: bad args: %w", err)
+	}
+	if a.Thought == "" {
+		return "", fmt.Errorf("reasoning_think: thought is required")
+	}
+	return "", nil
+}

toolsrv/server.go

@@ -201,6 +201,17 @@ Returns tools with descriptions, one per line.`,
 				"properties": {}
 			}`),
 		},
+		{
+			Name: "reasoning_think",
+			Description: `Scratchpad for externalizing reasoning. Recorded in history but not shown to the user.`,
+			InputSchema: json.RawMessage(`{
+				"type": "object",
+				"required": ["thought"],
+				"properties": {
+					"thought": {"type": "string", "description": "Your reasoning."}
+				}
+			}`),
+		},
 	}
 
 	if e.toolRegistry != nil && e.sessionID != "" {