The plugins declare their vocabulary

• Context: the builtin tables named the GitHub and Slack plugins’ facts, effects, roles and steps, so a new source needed an edit in the core.
• Decision: internal/intent/builtin declares the core’s rows and imports no plugin’s domain; builtin.Merge maps each plugin’s [source] table onto the tables before the configuration and the profile load, for every plugin found, enabled or not.
• Consequence: a source plugs in with its manifest; a disabled plugin’s keys still parse.
• Enforced by: TestBuiltinHasNoProviderRows.