Add a New Issue Integration

New issue trackers and calendar integrations should be issue-provider plugins. A plugin keeps provider-specific configuration, API calls, and data mapping outside the core app and receives a stable plugin:<plugin-id> provider key.

Start with the repository's plugin-first integration guide. It covers the current IssueProviderPluginDefinition, manifest, credential storage, packaging, and verification requirements. Current bundled providers under packages/plugin-dev/ are the implementation examples.

Do not add a provider to src/app/features/issue/providers/, core unions, defaults, or configuration forms unless a maintainer has approved a core-only requirement that the plugin API cannot meet. Existing built-in providers remain in core for maintenance only.