Set up signed commits

Set up cryptographic commit signing for a [macOS / Linux / Windows] developer machine connected to a GitHub repository. Walk through the complete setup for GPG signing: generating a new GPG key pair with an appropriate algorithm and key size, exporting and adding the public key to GitHub, configuring Git globally to always sign commits and tags using the new key, verifying the signature on a test commit locally, and troubleshooting the five most common issues such as GPG agent not running, wrong key configured, or passphrase prompts failing in terminal. Include the equivalent steps for SSH signing as an alternative.