Add threadless terminal session plumbing

[ymichael]

Summary

• allow terminal sessions to be persisted without a thread owner while preserving existing thread terminal behavior
• add environment-scoped terminal API routes and a threadless websocket attach path
• relax daemon terminal.open threadId, add client helpers, and cover threadless terminal create/list/attach behavior

Validation

• pnpm exec turbo run typecheck --filter=@bb/db
• pnpm exec turbo run typecheck --filter=@bb/server-contract
• pnpm exec turbo run typecheck --filter=@bb/host-daemon-contract
• pnpm exec turbo run typecheck --filter=@bb/server
• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run test --filter=@bb/db -- --run test/data/terminal-sessions.test.ts test/migrate.test.ts test/schema.test.ts
• pnpm exec turbo run test --filter=@bb/server-contract -- --run test/contract.test.ts
• pnpm exec turbo run test --filter=@bb/host-daemon-contract -- --run test/contract.test.ts
• pnpm exec turbo run test --filter=@bb/server -- --run test/public/public-thread-terminals.test.ts
• pnpm exec turbo run test --filter=@bb/app -- --run src/hooks/cache-owners/cache-owner-registry.test.ts
• git diff --check