Add test for invalid `--max-diff-size` flag

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Task summary

The suggest command validates --max-diff-size and exits with "Invalid --max-diff-size value. Expected a positive integer." when given invalid input, but there is no test covering non-numeric, zero, or negative values.

Why this is a good first issue

The error message and validation logic already exist. The task is writing one CLI test that asserts exit code and stderr output — a good intro to the project's node:test patterns.

Suggested files or areas

• src/commands/suggest.ts — validation logic (line ~146)
• tests/empty-repo-suggest.test.mjs — pattern to follow
• tests/e2e/suggest-smoke.test.mjs — has a valid --max-diff-size case for reference

Acceptance criteria

• Test covers at least one invalid value (e.g. 0, -1, or abc)
• Test asserts non-zero exit code
• Test asserts the error message is printed
• npm test passes

Extra context

tests/config-store.test.mjs already tests invalid maxDiffSize values at the config layer; this issue covers the CLI flag path specifically.