Add shared types package

Shared TypeScript types, Zod validators, API contract definitions,
and constants used by both server and UI. Covers agents, goals,
issues, projects, and activity entities.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/shared/src/api.ts

@@ -0,0 +1,10 @@
+export const API_PREFIX = "/api";
+
+export const API = {
+  health: `${API_PREFIX}/health`,
+  agents: `${API_PREFIX}/agents`,
+  projects: `${API_PREFIX}/projects`,
+  issues: `${API_PREFIX}/issues`,
+  goals: `${API_PREFIX}/goals`,
+  activity: `${API_PREFIX}/activity`,
+} as const;