Add non-git fallback snapshotting for git_changed_files

[mmprotest]

Motivation

• Ensure change/progress detection works in non-git sandboxes by avoiding a silent “no changes” when git status fails.
• Provide a lightweight in-memory fallback so verification, patch sanity, and autonomy progress tracking see real file edits in empty or non-git workspaces.

Description

• Replaced git_changed_files(repo) in villani_code/state_runtime.py to preserve git status --short behavior when it succeeds and fall back to a filesystem snapshot diff when it fails.
• Added a module-level in-memory snapshot store _NON_GIT_SNAPSHOT_BY_REPO keyed by absolute repo path and a _snapshot_workspace_files() helper that records per-file fingerprints (size, mtime_ns) using relative POSIX-style paths and the existing is_ignored_repo_path logic.
• Fallback diff reports added, modified, and deleted relative paths (sorted) and updates the baseline after each call; on first non-git call the baseline is created and [] is returned.
• Kept changes local to state_runtime.py, reused the repo ignore rules, avoided disk persistence, and did not modify autonomy, verification or stop logic.

Testing

• Ran the focused tests for the new behavior with pytest -q tests/test_state_runtime.py -k git_changed_files and all relevant cases passed (7 passed).
• Ran the full state runtime test file with pytest -q tests/test_state_runtime.py and all tests passed (39 passed).

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