fix(cli): warn instead of failing the gate when the PR comment cannot be posted

[royalpinto007]

Fixes #24

Summary

On fork PRs the github.token is read-only, so the comment POST returns 403. upsertComment threw, the error escaped the top-level handler, and the gate exited 2 even when the comparison found no regression. Fork contributions are exactly where this bites.

Changes

• cmdCompare: wrap upsertComment in try/catch, log a warning with the reason, keep the comparison verdict.
• cli/index.ts: guard the module-level main() behind an entry-point check so tests can import cmdCompare.
• tests/comment-failure.test.ts: stubs fetch to 403 and asserts exit code 0 on a clean comparison with comment enabled.

Test plan

• npm test (69 passed, 10 files)
• npm run typecheck
• npm run lint

[github-actions]

evalgate: support-agent

PASS - No regressions detected.

Overall score: 94.2% (base) -> 94.2% (head) = 0.0pp

Case	Base	Head	Delta	Change
greeting-exact	100.0%	100.0%	0.0pp	flat
refund-intent-json	100.0%	100.0%	0.0pp	flat
mentions-policy	100.0%	100.0%	0.0pp	flat
order-id-format	100.0%	100.0%	0.0pp	flat
paraphrase-quality	86.0%	86.0%	0.0pp	flat
tone-rubric	100.0%	100.0%	0.0pp	flat
judge-helpfulness	73.6%	73.6%	0.0pp	flat

All cases unchanged.

tolerance 1.0% - worst case 0.0pp

_{Posted by evalgate - the build fails when your prompt gets dumber.}