Fix non-GitHub SSH remote validation

[simonyang08]

Closes #982

Summary

• accept GitHub SCP-style and ssh:// remote URLs
• reject SSH remotes whose host is not GitHub
• keep existing HTTPS, branch, and nested skill-path handling unchanged

Root cause

Non-GitHub SSH remotes were treated as plain owner/repository strings. That allowed them to reach the GitHub API and fail later with a misleading 404 instead of reporting that the host is unsupported.

User impact

Users now get an immediate, accurate validation error for unsupported SSH hosts. Existing GitHub SSH forms such as git@github.com:owner/repo.git and ssh://git@github.com/owner/repo.git continue to work.

Validation

• vendor/bin/pest tests/Unit/Skills/Remote/GitHubRepositoryTest.php - 32 passed, 79 assertions
• vendor/bin/phpstan analyse src/Skills/Remote/GitHubRepository.php --no-progress - passed
• full Pest suite - 1,012 passed; 8 unrelated WSL/configuration failures; 1 todo; 10 skipped
• git diff --check - passed

The full-suite failures involve existing wsl.exe and configuration expectations and do not touch the changed files.

AI disclosure

This change was prepared with OpenAI Codex assistance and reviewed and validated locally by the contributor.

[github-actions]

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