Feature: Log to file for GraphServer CLI

[griffinmilsap]

Closes #246

Summary

Add file logging for the ezmsg start / ezmsg serve service envelope.

This change adds --log-file support to both commands, passes the option through from start to the spawned serve subprocess, and otherwise falls back to EZMSG_LOG_FILE or a default per-run log path under a platform-appropriate ezmsg data directory. Default logs are organized by GraphServer port and use a timestamped filename for each execution.

Scope

This intentionally stays narrow and only affects the start / serve workflow. It does not attempt to define broader logging behavior yet for ez.run, GraphRunner, subprocess/session aggregation, or remote clients.

Testing

• Added one parser test for serve --log-file
• Added one command test verifying start passes --log-file through to serve
• Ran:

uv run pytest tests/test_command.py -q