feat(ssh): route GitHub SSH over port 443

[perezdap]

What

Adds an opt-out (Y/n, default yes) step to the identity setup that writes a Host github.com → ssh.github.com:443 block to ~/.ssh/config.

Why

On some networks (corporate firewalls, ISPs, VPNs) outbound SSH on port 22 is intermittently blocked, throttled, or reset — surfacing as random Connection timed out / Connection reset on git fetch/push, while HTTPS (443) stays reliable. GitHub officially supports SSH over its ssh.github.com:443 endpoint, and routing through it sidesteps the problem without changing remotes or keys.

Changes

• setup.ps1 — new Set-GitHubSSHOverHttps helper + auto-run prompt in Set-GitIdentity.
• setup.sh — mirrored configure_github_ssh_over_https helper + prompt in configure_identity.
• Tests: tests/Test-GitHubSSHOverHttps.ps1 and tests/test_github_ssh_over_https.sh (content-verification, matching existing style).

Safety

• Idempotent — skips if a Host github.com block already exists in ~/.ssh/config, so re-runs never duplicate or clobber.
• Composes with the existing core.sshCommand -i <key> setup: key selection and host/port rewrite are orthogonal.

Verification

• New + existing tests pass in both shells.
• setup.ps1 parses clean; setup.sh passes bash -n.
• Bash helper idempotency confirmed in a temp HOME: appends once, skips on re-run, exactly one Host github.com block.