Add a plugin-based issue provider architecture that allows plugins to
register as issue providers alongside the existing built-in providers.
- Plugin API: types for search, display, comments, two-way sync, and
field mappings in @super-productivity/plugin-api
- Registry service to manage plugin provider lifecycle
- HTTP proxy with SSRF protection for plugin network requests
- Adapter service implementing IssueServiceInterface for plugins
- Sync adapter for plugin-based two-way sync
- Plugin config UI in the issue provider edit dialog
- Plugin providers shown in setup overview and issue panel
Migrate GitHub from built-in to plugin as first proof:
- Bundled github-issue-provider plugin with full feature parity
- Reducer migration converts old GitHub data to plugin format while
preserving legacy fields for cross-version compatibility
- Auto-enable plugin when existing GitHub providers are detected
- Plugin registers under 'GITHUB' key via MigratedIssueProviderKey
- GitHub translations moved to plugin i18n bundles
Mark Two-Way Sync as experimental in the UI.
