Reject invalid numeric CLI flags

[vsolano9]

What & why

Bad numeric flags currently become NaN, silently skip compaction, or print $NaN while exiting successfully. This makes all three numeric options fail clearly before payload processing while preserving a zero token budget.

Closes #5

Changes

• --max, --max-tool, and --price now share one finite non-negative parser.
• Errors name the exact flag and received value, including a missing value.
• Real CLI subprocess tests cover each flag, infinity, missing input, and successful --max 0.
• README and the Unreleased changelog document the corrected contract.

Validation

• RED: all five invalid subprocess cases exited 0, while --max 0 passed.
• npm test passes 12/12 and repeated successfully 20 consecutive times.
• The complete suite passes on the supported minimum Node 18 and on Node 20.
• npm pack --dry-run includes only the intended five distributable files.
• git diff --check passes.
• The package has zero dependencies and no lockfile, so npm audit is not applicable.

Checklist

• node --test passes
• Change remains zero-dependency and deterministic
• Documentation and changelog are current
• Linked the related issue