test: add hermetic Git E2E baseline

[oflynned]

Summary

• execute the packaged action against temporary working and bare Git repositories
• intercept only GitHub push URLs, keeping the suite offline and credential-free
• characterize fresh, patch, minor, major, input, identity, branch, and push-failure behavior
• document issue Not seeing minor version bumped when feat: is used #122 with a skipped realistic webhook-payload regression
• run E2E and Docker smoke gates in CI

Verification

• npm run lint
• npm run typecheck
• npm test -- --runInBand (51 passing)
• npm run test:e2e (7 passing, 1 skipped regression)
• docker build -t android-version-bump:e2e-baseline .

Stacked on #125 so the test commit remains isolated from the runtime modernization.