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/files.ts

@@ -0,0 +1,13 @@
+import { mkdtempSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+/**
+ * Create a temporary file with the given content
+ */
+export function createTempFile(content: string, filename = "temp.txt"): string {
+  const tempDir = mkdtempSync(join(tmpdir(), "pullfrog-"));
+  const filePath = join(tempDir, filename);
+  writeFileSync(filePath, content);
+  return filePath;
+}