test: add test directory with sample files

test-dir/AGENTS.md

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+## Root Level Rules
+
+- Root rule 1
+- Root rule 2

test-dir/another.ts

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+export const config = {
+  strict: true
+}

test-dir/nested/AGENTS.md

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+## Nested Level Rules
+
+- Nested rule 1 (더 specific)
+- Nested rule 2

test-dir/nested/deep.ts

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+export const deep = true

test-dir/test.ts

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+export function greet(name: string): string {
+  return `Hello, ${name}!`
+}