Lists all available trigger sources (raw + enriched), across providers:

• pipedream — Pipedream component catalogue (compile-time loaded).
• native — V7-managed native trigger skills (category: :trigger
  rows in the skills table).

Native list items include top-level transport metadata:
delivery_modes, preferred_delivery_mode, and transport_capabilities.

Returns a compact listing without full provider_data. Use the
detail endpoint (GET /api/trigger_sources/:provider/:provider_key)
when you need provider_data (especially for native sources, where
it carries fetch_skill_id and arg_specs).

Can be filtered by integration_id and/or provider.