Use ~/.gua state dir and record daemon intervals

[GHoon-Lee]

Summary

• move default gua runtime state to ~/.gua/gua.db, ~/.gua/gua.pid, and ~/.gua/gua.log
• treat the default DB as appendable local history while keeping custom --db paths collision-safe
• record daemon run intervals and attach samples to run metadata so reports can compute GPU-hours without repeating --interval
• update docs, changelog, and tests for the new default path and interval behavior

Verification

• uv run python -m pytest -q
• uv run ruff check
• uv run ruff format --check
• uv run python -m mypy
• uv build --out-dir /tmp/gua-dist-home-default
• bash scripts/smoke-dist-wheel.sh /tmp/gua-dist-home-default/gpu_usage_audit-1.0.2-py3-none-any.whl