Refactor action with agent interface system and make anthropic_api_key optional

- Created extensible agent interface with install() and execute() methods
- Moved Claude Code logic to agents/claude.ts implementing Agent interface
- Added utilities directory for reusable functions (exec, files)
- Refactored index.ts to be minimal (35 lines) using agent abstraction
- Made anthropic_api_key optional in action.yml
- Updated Node.js imports to use node: prefix convention
- Bumped version to 0.0.5
- Architecture now supports multiple agents (OpenAI, Gemini, etc.)

agents/types.ts

@@ -0,0 +1,34 @@
+/**
+ * Standard interface for all Pullfrog agents
+ */
+export interface Agent {
+  /**
+   * Install the agent and any required dependencies
+   */
+  install(): Promise<void>;
+
+  /**
+   * Execute the agent with the given prompt
+   * @param prompt The prompt to send to the agent
+   * @param options Additional options specific to the agent
+   */
+  execute(prompt: string, options?: Record<string, any>): Promise<AgentResult>;
+}
+
+/**
+ * Result returned by agent execution
+ */
+export interface AgentResult {
+  success: boolean;
+  output?: string;
+  error?: string;
+  metadata?: Record<string, any>;
+}
+
+/**
+ * Configuration for agent creation
+ */
+export interface AgentConfig {
+  apiKey?: string;
+  [key: string]: any;
+}