Windows support: replace fcntl with cross-platform locking

[hirobirofran]

Summary

I'd like to use intercom on Windows, but it currently doesn't work due to fcntl usage, which is a POSIX-only module.

What doesn't work

• fcntl.flock() in _acquire_lock(), _is_alive(), and _append_event() — fcntl doesn't exist on Windows
• tmux dependency — not available natively on Windows, but this is already handled by the AGENT_NAME fallback, so it's a non-issue

Possible fix

The fcntl-based locking could be replaced with a cross-platform approach, for example using filelock, which abstracts over both POSIX and Windows with a simple API.

I'm happy to work on a PR if this direction seems reasonable to you.

[odfalik]

Thanks for the thoughtful writeup, @hirobirofran! Yes, PR welcome.

One nuance to flag before you start: _acquire_lock() writes the PID and optional AGENT_NAME into the lock file itself, and _read_static_name() reads it back later. filelock treats lock files as opaque (necessary for its Windows msvcrt backend), so dropping it in would also force splitting per-agent state into two files (agent.lock + agent.info).

To keep the on-disk layout and avoid a new dependency, I'd rather go with a small internal shim: fcntl on POSIX, msvcrt.locking on Windows, ~25 lines, same API as today. Happy to take a PR in that shape.

The three call sites that need updating: _acquire_lock(), _is_alive(), and _append_event() (all in intercom/server.py). Tests would be great too if you have the cycles.