Preserve proof surface status paths

[Reedtrullz]

Summary

• Fix proof-surface generated readback path parsing for modified files that start with a leading porcelain status space.
• Add a focused regression assertion so docs/dashboard.md does not render as ocs/dashboard.md.

Verification

• python3 -m pytest tests/test_proof_surface.py -q -> 3 passed
• python3 -m compileall -q agent_os/proof_surface.py tests/test_proof_surface.py
• git diff --check

Non-claims

• No behavior change to CI proof recording semantics.
• No deploy/provider/remote-worker/browser automation changes.

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