integrations: add portable Supabase descriptor

[EmersonBraun]

Outcome

Add a portable Supabase integration descriptor to the public @agentskit/integrations package.

Acceptance criteria

• Stable provider id and public documentation metadata.
• Public authentication metadata with least-privilege credential guidance.
• Safe public fields and scopes.
• At least one read-only account/project validation action.
• listIntegrations() returns the descriptor.
• Unit tests use synthetic credentials and mocked provider responses.
• Errors are typed and actionable.

Privacy boundary

This issue is specified solely through the public provider API. It must not reference or encode any private consumer setup flow, registry, preflight behavior, credential storage, runtime, fixture, or business policy.