feat: add ToolRegistry.clone() and replace() for per-session registries

src/tools/registry.ts

@@ -31,6 +31,26 @@ export class ToolRegistry {
     return this.tools.delete(name);
   }
 
+  /** Replace an existing tool with a new implementation. Throws if not registered. */
+  replace(tool: Tool): void {
+    if (!this.tools.has(tool.name)) {
+      throw new Error(`Tool '${tool.name}' is not registered — cannot replace`);
+    }
+    this.tools.set(tool.name, tool);
+  }
+
+  /** Create a shallow clone of this registry (new Map, same Tool objects + policy). */
+  clone(): ToolRegistry {
+    const cloned = new ToolRegistry();
+    for (const tool of this.tools.values()) {
+      cloned.register(tool);
+    }
+    if (this._policy) {
+      cloned.setPolicy(this._policy);
+    }
+    return cloned;
+  }
+
   get(name: string): Tool | undefined {
     return this.tools.get(name);
   }