feat(vision): add the configurable HTTP vision provider

Registers an OpenAI-compatible chat-completions provider into
ctx.imageRecognition with a user-editable baseURL + API key via a settings
section and the credential plane. Logs the secret-free vision request body
as the image-recognition/llm-request session event.

Co-Authored-By: Claude <noreply@anthropic.com>

packages/core/session/src/known-event-types.ts

@@ -61,4 +61,5 @@ export const KNOWN_SESSION_EVENT_TYPES: ReadonlySet<string> = new Set([
   'turn/start',
   'user/message',
   'web/deepseek-search-llm-request',
+  'image-recognition/llm-request',
 ])