chore: add CI — lint SKILL.md frontmatter and smoke-test all scripts

Problem

The repo has no .github/ directory — zero CI, zero tests. Issues #13 and #14 (wrong throughput sort, wrong modality filter) have been shipping silently because nothing catches regressions before merge.

What CI should do

Frontmatter lint — verify every SKILL.md has required fields:

name: <non-empty string>
description: <non-empty string>

Script smoke test — for every script in */scripts/*.ts, verify:

1. npm install completes without error
2. Running the script with no args (or --help) exits cleanly (not with a stack trace)

Optionally: a fixture test for list-models.ts and compare-models.ts that mocks the API response and asserts the sort output is correct — this would have caught #13 immediately.

Why now

The throughput sort bug (#13) is a correctness issue that produces silently wrong agent behavior. With no CI, similar bugs can land undetected. The fix is small — a single GitHub Actions workflow file.

Reviewed by Perry