fix: stop LocalBackend execute hanging on Windows date/time

[khuzaymahbinharis-jpg]

Summary

• Close stdin with DEVNULL for foreground LocalBackend.execute() and async_execute(), matching the existing background-process behavior.
• Prevent interactive Windows shell built-ins such as date and time from blocking until the execution timeout.
• Add regression coverage for both synchronous and asynchronous execution.

Fixes #103

Why

On Windows, commands run through cmd /c. Bare date and time are interactive commands: after displaying the current value, they wait for user input.

LocalBackend does not expose stdin to callers, so foreground executions should be non-interactive. Passing DEVNULL causes these commands to receive EOF immediately rather than hanging until timeout.

Testing

Passed:

• uv run ruff check .
• uv run ruff format --check .
• Sync and async DEVNULL regression tests
• Windows date / time regression tests
• Existing ordinary command execution coverage

On Windows, date and time now return immediately rather than waiting for the configured execution timeout.

The broader test/typecheck commands expose existing Unix-specific assumptions when run on Windows, including Unix commands/filesystem behavior such as sleep, pwd, os.chown, Unix permissions and /tmp. These failures are unrelated to this change.