refactor: Brand ToolResponse and narrow ToolEntry.call (P4 lock)

[jirispilka]

Part of #935. Depends on #937 (all handlers must route through respond* first).

Goal

Make an unclassified error a compile error — closes P4. This is the structural guarantee that distinguishes Option F from a plain helper.

Scope

• Brand the constructor output in src/utils/mcp.ts:

  declare const wireBrand: unique symbol;
  export type ToolResponse = MCPWireShape & { readonly [wireBrand]?: never };

  The respond* functions return ToolResponse; raw object literals don't carry the brand.

• Narrow the handler signature in src/types.ts (currently call: (args) => Promise<object>):

  call: (args: InternalToolArgs) => Promise<ToolResponse>;

• Fix whatever the compiler flags — those are the spots refactor: Adopt respond* response constructors across tools #937 missed (hand-rolled shapes or stray isError).

Acceptance

• A hand-rolled { content: [...] } or bare isError: true in a handler does not compile.
• pnpm run type-check, pnpm run lint, pnpm run test:unit clean.