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/factory.ts

@@ -0,0 +1,16 @@
+import { ClaudeAgent } from "./claude";
+import type { Agent, AgentConfig } from "./types";
+
+export type AgentType = "claude";
+
+/**
+ * Factory for creating agent instances
+ */
+export function createAgent(type: AgentType, config: AgentConfig): Agent {
+  switch (type) {
+    case "claude":
+      return new ClaudeAgent(config);
+    default:
+      throw new Error(`Unsupported agent type: ${type}`);
+  }
+}