feat(server): extract question httpapi contract into packages/server

[nexxeln]

summary

• move the experimental question HttpApi contract into packages/server and export it from the server definition surface
• generate packages/server OpenAPI from the extracted contract while keeping temporary transport-local schemas cycle-free
• switch the existing opencode question handler to consume the extracted contract without changing runtime behavior

testing

• bun typecheck in packages/server
• bun typecheck in packages/opencode
• bun test test/server/question-httpapi.test.ts --timeout 30000 in packages/opencode