Add sandbox filesystem methods (list_dir, mkdir, remove, upload,
download, stream_upload, stream_download) and interactive PTY sessions
(PtySession, AsyncPtySession) with reconnect support per
FILE_TERMINAL.md spec. Refactor error handling into exceptions.py as
shared handle_response(). Replace API-key-only proxy auth with unified
_proxy_headers() supporting both API key and JWT. Fix stream_upload to
build multipart manually instead of relying on httpx files= with
generators. Switch Makefile SPEC_URL from main to dev branch. Regenerate
models from updated OpenAPI spec (adds teams, channels, metrics, PTY
endpoints). Add comprehensive unit and integration tests. Trim AGENTS.md
to verified facts only.
