feat(studio): add tool execution and hide AI thinking tags

Changes:
- Hide <think> tags from user in Studio chat
- Add tool call detection [TOOL_CALL:{...}] in AI responses
- Execute crush tool when requested by AI
- Show loading animation while AI is thinking

The AI can now:
1. Respond directly to user
2. Request tool execution via [TOOL_CALL:{"tool":"crush","task":"..."}]

The system automatically executes the tool and includes results.

Assisted-by: MiniMax-M2.7 via Crush <crush@charm.land>

internal/api/handlers_tools_exec.go

@@ -0,0 +1,80 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"os/exec"
+	"strings"
+)
+
+type toolCallRequest struct {
+	Tool string `json:"tool"`
+	Task string `json:"task"`
+}
+
+type toolResult struct {
+	Success bool   `json:"success"`
+	Output  string `json:"output"`
+	Error   string `json:"error,omitempty"`
+}
+
+func (s *Server) handleToolCall(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		writeError(w, "POST only", http.StatusMethodNotAllowed)
+		return
+	}
+
+	var req toolCallRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	if req.Tool != "crush" {
+		writeError(w, "unsupported tool: "+req.Tool, http.StatusBadRequest)
+		return
+	}
+
+	if req.Task == "" {
+		writeError(w, "task is required", http.StatusBadRequest)
+		return
+	}
+
+	result := executeTool(req.Tool, req.Task)
+	writeJSON(w, result)
+}
+
+func executeTool(tool, task string) toolResult {
+	var cmd *exec.Cmd
+
+	switch tool {
+	case "crush":
+		cmd = exec.Command("crush", "run", task)
+	default:
+		return toolResult{Success: false, Error: "unknown tool: " + tool}
+	}
+
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		return toolResult{
+			Success: false,
+			Output:  string(output),
+			Error:   err.Error(),
+		}
+	}
+
+	return toolResult{
+		Success: true,
+		Output:  string(output),
+	}
+}
+
+func buildToolMessage(tool, task string, history []string) string {
+	var b strings.Builder
+	b.WriteString("TASK: " + task + "\n\n")
+	b.WriteString("CONVERSATION HISTORY:\n")
+	for _, msg := range history {
+		b.WriteString(strings.Repeat(" ", 4) + strings.Join(strings.Split(msg, "\n"), "\n"+strings.Repeat(" ", 4)) + "\n")
+	}
+	return b.String()
+}