User resolution - improve committer mapping and robustness

Problem

CLA-Bot currently matches committers by fullname, which is fragile: homonyms, renames or edited fullnames can force unnecessary re-signing.

Optional improvements to increase matching robustness:

Use GPG commit signatures (when present) to obtain a verified email address.
Perform a combined check (email + fullname) to reduce false positives on homonyms.

Consider privacy and retention implications when storing emails (GDPR).

Improves matching accuracy and reduces unnecessary re-signing. Introduces design decisions about what personal data to store and how long to keep it.