[feature request] Support GPG/SSH-signed commits to enable “Verified” status on GitHub

**Is your feature request related to a problem? Please describe.**
Currently, every commit that Salesforce DevOps Center makes to our GitHub repository is unsigned, so GitHub always shows “Unverified commit.” This prevents teams from using standard GPG/SSH-based verification and breaks compliance or security policies that require cryptographically signed commits.

**Describe the solution you’d like**
Provide an option in the DevOps Center UI to register a user’s GitHub-associated GPG or SSH key, and have all automatic commits execute `git commit -S` (or equivalent SSH signing). If signing fails (missing key, passphrase needed), DevOps Center should surface a clear error message.

**Additional context**
Many competing tools (e.g., Copado, Gearset) offer commit-signing to satisfy enterprise security requirements. Enabling signed commits in DevOps Center would allow organizations to enforce signed-commit policies, improve auditability, and integrate seamlessly with GitHub branch protection or CI gates that reject unsigned commits.

ref. https://docs.copado.com/articles/#!copado-ci-cd-publication/signing-git-commits-with-copado
ref. https://docs.gearset.com/en/articles/7865956-commit-signing

The issue names no repository files, tests, or entry points. Start by locating the DevOps Center UI for GitHub key registration and the automatic commit flow; done means supported GPG or SSH signing, clear failure reporting, and GitHub showing the resulting commits as verified.

Written by the indexing model from the issue text.