Add more tests for FileSystemParser

And update agents.md for new instructions related to writing tests

Author: scriptype

AGENTS.md

@@ -2,3 +2,13 @@
 - Avoid semicolons unless it's absolutely necessary
 - Always use block statements even if it could be a one-liner
 - Always use curly braces {} for control structures (if, for, while, etc.), even if the body is a single statement
+
+## Test Assertions Formatting
+- Assertion function call on one line (e.g., `t.ok(`, `t.equal(`, `t.notOk(`)
+- Actual/expected value on separate line
+- Message on its own line
+- Add blank line between consecutive assertions
+- Only use `t.plan()` for async or non-deterministic tests where assertions might be skipped; omit for sync tests
+
+## Test File Operations
+- Use `Promise.all()` to parallelize independent async file operations (mkdir, writeFile, etc.) instead of awaiting them serially