feat: add tested latest stable branch selector

[vitormattos]

Extracts nightly stable-branch selection into a small, deterministic Action.

The Action:

• accepts only exact stable<N> refs when discovering release lines;
• compares numerically, so stable100 sorts after stable35;
• ignores suffix branches such as stable36-test;
• exposes current/latest branch and release-line outputs plus is-latest;
• prints the resolved values in logs and the job summary;
• fails closed when no valid stable branch exists.

Unit coverage exercises numeric ordering, exact-name filtering, non-stable refs and empty repositories. VERSION moves to 0.6.0 because this adds a public Action.