fix: trigger threshold exit when waste ratio equals threshold

[HeaTTap]

Fixes #16

What & why

Previously _maybe_fail used ratio > threshold without equality, so a waste ratio exactly at the threshold never triggered the gate exit. This updates the check to ratio >= threshold so thresholds behave as expected.

Changes

• Changed ratio comparison in _maybe_fail from ratio > threshold to ratio >= threshold in src/ctxlens/cli.py
• Added unit tests in tests/test_cli.py verifying _maybe_fail at exact threshold raises typer.Exit with EXIT_THRESHOLD and values below threshold or None do not raise

Checklist

• Focused change with a clear description
• Ran format / lint / tests (see README)
• Updated docs or CHANGELOG if relevant
• Linked any related issue (Closes --fail-over-ratio never fires at exactly the threshold #16)