feat(shared): add core domain types

Pod/Engineer, EngineerContext (live pre-push signal), Collision, and
Intervention — the type contract between frontend and backend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shared/src/index.ts

@@ -0,0 +1,14 @@
+export type { Pod, Engineer } from './pod.js';
+export type { EngineerContext } from './engineer.js';
+export type {
+  Collision,
+  CollisionSeverity,
+  GithubStateSnapshot,
+} from './collision.js';
+export type {
+  Intervention,
+  InterventionKind,
+  InterventionStatus,
+  SuggestedAction,
+  SuggestedActionKind,
+} from './intervention.js';