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.)

utils/exec.ts

@@ -0,0 +1,15 @@
+import { exec } from "node:child_process";
+import { promisify } from "node:util";
+
+export const execAsync = promisify(exec);
+
+/**
+ * Execute a shell command with optional environment variables
+ */
+export async function executeCommand(
+  command: string,
+  env?: Record<string, string>
+): Promise<{ stdout: string; stderr: string }> {
+  const execEnv = env ? { ...process.env, ...env } : process.env;
+  return execAsync(command, { env: execEnv });
+}