Extract todo system into standalone packages/todos/ package #69

What to build

The todo system is well-separated conceptually but lives across three locations that each have conflict risk: packages/shared/src/todoStore.ts, packages/contracts/src/todos.ts, and apps/server/src/todoPersistence.ts. Move all todo logic into a standalone packages/todos/ package with its own subpath exports, contracts, and server layer. This isolates all fork-specific todo enhancements from upstream conflicts.

Move plan

• packages/shared/src/todoStore.ts → packages/todos/src/store.ts
• packages/contracts/src/todos.ts → packages/todos/src/schema.ts
• apps/server/src/todoPersistence.ts → packages/todos/src/persistence.ts
• Register RPC handlers via the extension registry from Create an RPC extension registry for WebSocket handlers #68
• Web-side TodoPanel.tsx and useTodos.ts import from @t3tools/todos instead of @t3tools/shared/todoStore
• Remove todo exports from packages/shared/package.json subpath exports

Acceptance criteria

• packages/todos/ exists with package.json, tsconfig.json, and proper subpath exports
• @t3tools/todos/store, @t3tools/todos/schema, @t3tools/todos/persistence import paths work
• Todo panel in web app continues to function identically
• vp check passes
• vp test passes (including any todo-related tests)
• packages/shared/package.json no longer references todo modules

Blocked by

• Resolve upstream merge conflicts from t3-code/main fork #67 (Resolve upstream merge conflicts)
• Create an RPC extension registry for WebSocket handlers #68 (RPC extension registry)