feat(04-02): implement _handle_use_project with context tracking

- Add _handle_use_project method
- Update user context when switching projects
- Return error card with Create button for non-existent projects
- Route use command in handle_message

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gsd-openclaw/src/gsd_openclaw/orchestrator.py

@@ -136,6 +136,60 @@ class Orchestrator:
                 content=str(e)[-self.MAX_OUTPUT_LENGTH:]
             )
 
+    def _handle_use_project(self, user_id: str, project_name: str) -> Dict[str, Any]:
+        """Handle /gsd:use command.
+
+        Switches the user's current project context. Shows error with
+        "Create New Project" button if the project doesn't exist.
+
+        Args:
+            user_id: ID of the user sending the command.
+            project_name: Name of the project to switch to.
+
+        Returns:
+            Feishu Card JSON dict as response.
+        """
+        # Validate project name
+        if not project_name or not project_name.strip():
+            return self.formatter.format_error(
+                title="Project name required",
+                content="Please provide a project name: /gsd:use <name>"
+            )
+
+        project_name = project_name.strip()
+
+        try:
+            # Check if project session exists
+            session = self.tmux_manager.get_session(project_name)
+
+            if session is not None:
+                # Project exists - update user context and return success
+                user_ctx = self._get_user_context_manager(user_id)
+                user_ctx.set_current_project(project_name)
+
+                return self.formatter.format_success(
+                    title="Project switched",
+                    content=f"Switched to project '{project_name}'"
+                )
+            else:
+                # Project doesn't exist - return error with Create button
+                create_button = CardButton(
+                    text="Create New Project",
+                    value={"action": "new-project", "name": project_name},
+                    button_type="primary"
+                )
+                return self.formatter.format_error(
+                    title="Project not found",
+                    content=f"Project '{project_name}' does not exist",
+                    buttons=[create_button]
+                )
+
+        except Exception as e:
+            return self.formatter.format_error(
+                title="Switch failed",
+                content=str(e)[-self.MAX_OUTPUT_LENGTH:]
+            )
+
     def parse_command(self, message: str) -> Optional[Tuple[str, str]]:
         """Parse a message for /gsd:* command pattern.
 
@@ -192,6 +246,8 @@ class Orchestrator:
             # Route special commands
             if command == 'new-project':
                 return self._handle_new_project(user_id, args)
+            if command == 'use':
+                return self._handle_use_project(user_id, args)
 
             # Get project name from context
             project_name = context.get("project", "default")