Apply coding best practices: strict mypy, remove future annotations, fix async concurrency, hoist imports

[Copilot]

Applies coding best practices across the codebase: strict mypy compliance, proper import placement, removal of unnecessary from __future__ import annotations, and fixing a sequential async pattern that prevented concurrent execution.

mypy strict: 7 errors → 0

• _protocol.py: explicit int annotation on decode_header to fix no-any-return
• worker.py, remote.py: assert guards before accessing nullable _sandbox
• remote.py: TYPE_CHECKING import for DockerSandbox forward reference
• redis.py: type: ignore[import-not-found] for optional dependency

Remove from __future__ import annotations

Removed from docker.py and guest_agent.py. Project targets >=3.10 so PEP 604 unions work at runtime. String annotations already in place where needed (__aenter__).

Hoist imports

• __main__.py: import shutil moved to top (was duplicated inline in 3 CLI handlers)
• docker.py: _progress_callback import moved to top (no circular dep risk)

Fix async concurrency in .map()

_make_map_method was awaiting submissions and results sequentially, defeating the purpose of batch execution:

# Before — sequential
results = [await start_method(*args, **kwargs) for args in args_list]
return [await r for r in results]

# After — concurrent via gather
coros = [cast(Awaitable[object], start_method(*args, **kwargs)) for args in args_list]
results = await asyncio.gather(*coros)
awaitables = [cast(Awaitable[object], r) for r in results]
return list(await asyncio.gather(*awaitables))

Both phases (submit all tasks, then collect all results) now run concurrently.