Harden PytestPilot execution and repair attempts

[godkunzzz3]

What changed

• add explicit Local and Docker execution backends with network, filesystem, privilege, CPU, memory, pids, file-size, output, environment, and timeout controls
• enforce fresh source revisions at mutation time with path-bound SHA-256 read tokens and editable-path checks
• isolate every repair attempt in an independent Git worktree and only copy back a candidate after focused and full pytest pass
• record attempt patches, baseline commits, introduced failures, and selection state
• document the threat model and provide a minimal sandbox image

Why

Pytest previously ran inside the host permission boundary, source-read policy was only audited after mutation, and failed attempts polluted later attempts. These changes turn the three policies into runtime boundaries and keep failed patches away from the user's working tree.

Validation

• .venv/bin/python -m pytest tests -q: 903 passed, 2 skipped
• benchmark infrastructure tests: 37 passed
• git diff --check: passed
• Python compileall: passed

Known environment limitation

A real Docker smoke could not be run on the development host because the Docker CLI was not installed. Docker argument construction and boundary configuration are covered by focused tests.