AI services

AI services let a plugin own bb’s utility inference (thread titles, commit messages) and transcription.

The full walkthrough and the snippet are on the bb.experimental_aiServices card in Backend API. In short: registration happens on the server, the implementation of experimental_aiServicesHostContract lives in the host entry, the user selects through BB_INFERENCE/BB_TRANSCRIPTION, and core calls the primary host and applies its own retry/fallback policy: timeouts and rate limits fall back to the fallback model, authentication errors do not.