Make gua daemon run in background

[GHoon-Lee]

Summary

• Make gua daemon start the collector in the background by default.
• Keep gua daemon --foreground for systemd/debugging and add gua start, gua status, and gua stop.
• Make doctor recommendations, report headers, README examples, and wheel smoke checks use gua as the canonical command while keeping gpu-usage-audit as a compatibility entry point.

Tests

• uv run ruff check
• uv run ruff format --check
• uv run mypy
• uv run pytest