feat: Own settings management for GitHub PAT and default repo

[chubes4]

Problem

Currently GitHubAbilities reads github_pat and github_default_repo from Data Machine core's PluginSettings. Once the code is extracted, the extension needs to manage its own settings — either with its own options or by providing a settings registration hook.

Options

1. Own wp_options — datamachine_code_github_pat, datamachine_code_github_default_repo
2. Settings API hook — Register settings via datamachine_settings filter so they appear in core's settings UI
3. Hybrid — Read from own options, fall back to core's settings for migration

Acceptance Criteria

• GitHub PAT stored in extension-owned option
• Default repo stored in extension-owned option
• Migration path: if extension options are empty, read from core settings
• CLI: wp datamachine-code github set-pat <token>
• CLI: wp datamachine-code github set-repo <owner/repo>