Generate or update tests for $ARGUMENTS.
Read the source file at $ARGUMENTS. Generate production-like test scenarios covering its public surface -- happy path, key edge cases, and failure modes at the configured depth. Write or update the test file following the tailtest Step 4 rules in AGENTS.md (correct location, correct name, style-matched to existing tests). Run the tests and report only failures; stay silent if all pass.
Treat the file as new-file regardless of its git status -- this skill explicitly requests generation even for legacy files or files the Stop hook would normally skip.
After completing, update .tailtest/session.json: add the file to generated_tests and clear it from pending_files if present.