Update nix flake for v0.13.1

[wesm]

Summary

• Updated vendorHash to sha256-TfrbpW6x2g/dcE9VSAy3ktT3knalC0QQgI3q77RCqQc=
• Updated version to 0.13.1

Automated update via scripts/update-nix-flake.sh.

[roborev-ci]

roborev: Combined Review (a8654f0)

Clean: no Medium, High, or Critical findings were reported.

The reviewed diff only updates the Nix flake package version from 0.13.0 to 0.13.1; no code, dependency, build input, script, credential, or runtime behavior changes were identified.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

Note: gemini review skipped (agent quota exhausted)