Restore MQ loader check command

[Teichoui]

Summary

Restores the non-interactive redfetch check command used by MacroQuest loader integrations.

Why

Current upstream RedFetch treats check as an unknown command. That exits with code 2 and no stdout, matching the MQ loader log noise from issue RedGuides#29.

Details

• Adds a machine-readable check command back to the CLI.
• Writes only the update count to stdout for successful checks.
• Redirects incidental command output to stderr so loader parsing stays stable.
• Restores lightweight manifest-vs-local-version update checking without downloading files.
• Adds a first-run configuration preflight so the command does not open interactive setup from MQ.

Validation

• python -m py_compile src/redfetch/main.py src/redfetch/update_check.py src/redfetch/sync_types.py src/redfetch/config_firstrun.py
• python -m pytest tests/test_check.py tests/test_first_run_setup.py tests/test_sync_pipeline_regressions.py tests/test_path_reset.py

Related to RedGuides#29