RPC registry: migrate h-code RPC methods to register()

[harrryyd]

What to build

Migrate the five h-code RPC methods to use RpcMethodRegistry.register() instead of the static WS_METHODS / WsRpcGroup / RPC_REQUIRED_SCOPE / handler switch pattern:

• todo.load — schema in packages/contracts/src/todos.ts, handler in apps/server/src/ws.ts
• todo.mutate — same files
• mcp.listServers — handler in apps/server/src/ws.ts
• mcp.toggleServer — handler in apps/server/src/ws.ts
• review.getDiffPreview — handler in apps/server/src/ws.ts

Each method migrates by calling registry.register() from its domain file (or a dedicated registration file). The handler moves out of the giant switch in ws.ts into a standalone function.

This is the vertical-slice proof that the registry pattern works end-to-end: contracts → server handler → client types.

Acceptance criteria

• todo.load registered via registry.register() with schema, scope, and handler
• todo.mutate registered via same pattern
• mcp.listServers and mcp.toggleServer registered via same pattern
• review.getDiffPreview registered via same pattern
• Each handler is a standalone function (not a switch case in ws.ts)
• Old entries removed from WS_METHODS constant for these 5 methods
• Old entries removed from RPC_REQUIRED_SCOPE map for these 5 methods
• wsRpcScopes.test.ts still passes (checking remaining old entries)
• Todo panel, MCP toggle, and review diff preview still work end-to-end
• vp check and vp run typecheck pass

Blocked by

• RPC method registry: core + wire into ws.ts #133