Mocked end-to-end core flow test

[sij411]

Description

Prove the Input -> Core -> Actions boundary via one mocked end-to-end test.

Proposal

• Create core with local actor
• Feed ReceivedFollow and assert actions
• Feed UserCreateNote and assert actions
• Optionally feed DeliverySucceeded/Failed and assert state updates

Done when

• Single test covers Follow->Accept and Create(Note)
• No HTTP server, no real storage, no async required

Out of scope

• Real Linux runtime