feat: Add property-based testing with Hypothesis

[mark-e-deyoung]

Problem

The test suite uses example-based testing (specific inputs → specific outputs) which can miss edge cases. Property-based testing explores the input space automatically to find edge-case failures.

Solution

Add Hypothesis (well-established property-based testing framework, used by pytest and in academia/industry) and write property-based tests for:

• sanitize_git_url() — URL redaction invariants
• _get_backoff_delay() — monotonicity, bounds

Files

• requirements.txt — add hypothesis>=6.0
• .github/workflows/ci.yml — add hypothesis to pip install
• tests/unit/test_scm.py — Hypothesis tests for sanitize_git_url
• tests/unit/test_backoff_hypothesis.py — Hypothesis tests for backoff