fix: avoid duplicate check analysis

[Rayan-and-beyond]

Summary

• analyse each run once in check
• make strict exit behavior respect the selected severity filter
• clarify the strict help text and cover the regression

Closes #5

Validation

• focused regression: 1 passed
• PYTHONPATH=. uv run pytest -q — 55 passed
• uv run ruff check agentrace/cli.py tests/test_cli.py — passed
• git diff --check — passed

[github-actions]

Merged, @Rayan-and-beyond. Thank you for taking the time.

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