fix(deps): pin zod to v3 for MCP SDK type compatibility

The MCP SDK's TypeScript types expect Zod v3 class-based schemas.
zod@^4.x types are incompatible, causing CI typecheck failures.

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

Author: stephendolan

bun.lock

@@ -51,7 +51,7 @@
     "conf": "^15.0.2",
     "dayjs": "^1.11.19",
     "ynab": "^2.10.0",
-    "zod": "^4.3.6"
+    "zod": "^3.25.0"
   },
   "devDependencies": {
     "@biomejs/biome": "^2.3.11",