Update release metadata for v1.3.1

[robzolkos]

Summary

• stamp the Nix package version to 1.3.1
• stamp the Claude plugin version to 1.3.1

Validation

• GOWORK=off make release VERSION=1.3.1 DRY_RUN=1
• the full non-dry-run release preflight also passed before branch protection rejected the direct main push; the release script rolled back without pushing the tag

After this merges, rerunning the release script will tag the stamped commit and trigger the release workflow.

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Summary by cubic

Stamps the release version to 1.3.1 in the Nix package and Claude plugin metadata so the release script can tag this commit.

• Updates nix/package.nix and .claude-plugin/plugin.json from 1.3.0 to 1.3.1.
• After merge, rerun the release script to tag the stamped commit and trigger the release workflow.

^{Written for commit 9a2ee47. Summary will update on new commits.}

[Copilot]

Pull request overview

Updates stable-release metadata to version 1.3.1.

Changes:

• Bumps the Nix package and Claude plugin versions to 1.3.1.

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Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
nix/package.nix	Stamps Nix package version 1.3.1.
.claude-plugin/plugin.json	Stamps Claude plugin version 1.3.1.

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