fix: reject negative show character limits

[Gambit-Checkmate]

What & why

Closes #21. Negative show --max values currently reach Python's negative slicing and drop trailing characters instead of acting as a length limit. Validate the option during argument parsing and return a usage error (exit 2) before loading transcripts.

Changes

• Add a non-negative integer argument type for show --max.
• Preserve the default of 4000, positive limits, and zero.
• Clarify the CLI help and README.
• Add regression tests for negative limits (-1, -5), zero, positive limits, and the default.

Checklist

• Focused change with a clear description
• Ran format / lint / tests (see README)
• Updated docs or CHANGELOG if relevant
• Linked any related issue

Verification

Developed with AI assistance on Windows. The two negative-limit tests failed against the original implementation because transcript loading was reached; all 43 tests pass after the fix (baseline: 37).

Ruff lint and formatting checks pass for both changed Python files. The full repository lint check still reports 14 pre-existing diagnostics in unrelated files; the baseline full formatting check also flagged three unrelated files. Those files are intentionally untouched.