Provider authoring

A provider is an independently installed distribution that registers one implementation in the qg.providers entry-point group. It depends on quality-graph-core and must not import the CLI or another provider.

[project.entry-points."qg.providers"]
gitlab = "qg_gitlab:provider"

The loaded object satisfies quality_graph_core.Provider: it exposes a stable name and a generate(Graph) -> GeneratedProject method. Provider-specific configuration is available through Graph.provider.values; core validates its shape as data, while the provider owns its semantics and fail-closed validation.

Generated files must be deterministic, repository-relative, and complete. A provider should test its wheel independently, verify discovery from package metadata, and reject configuration for a different provider name. Provider releases must declare a compatible core version and document any generated-output migration.