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/collision.ts

@@ -0,0 +1,29 @@
+export type CollisionSeverity = 'info' | 'warn' | 'critical';
+
+/**
+ * A detected overlap between two or more engineers' live work, fused from
+ * vision-derived contexts and GitHub state.
+ */
+export interface Collision {
+  id: string;
+  podId: string;
+  /** File both engineers are touching. */
+  file: string;
+  /** Symbol-level overlap, if known. */
+  symbol?: string;
+  /** Engineer ids involved in the overlap. */
+  engineers: string[];
+  severity: CollisionSeverity;
+  /** Snapshot of relevant GitHub state at detection time. */
+  githubState?: GithubStateSnapshot;
+  detectedAt: string;
+}
+
+export interface GithubStateSnapshot {
+  /** Open branches touching the file, keyed by engineer/login. */
+  branches?: Record<string, string>;
+  /** Open PR numbers touching the file. */
+  openPrs?: number[];
+  /** Whether any involved engineer has unpushed local changes. */
+  unpushed?: boolean;
+}