chore: update @primer/react to version 38.15.0 and increment MCP package version to 0.3.1

[francinelucca]

Relates to https://github.com/github/primer/issues/6453

Changelog

Changed

• add @primer/mcp to next release step to prevent stable version from being published ahead of time

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

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Latest commit: 6d4874f

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[Copilot]

Pull request overview

Updates package versions in the monorepo to align @primer/react with the new 38.15.0 release and bumps @primer/mcp to 0.3.1, while also adjusting the release-candidate workflow so MCP isn’t published under @next with a stable (non-rc) version.

Changes:

• Bump @primer/react to 38.15.0 (including workspace/example lockfile entries).
• Bump @primer/mcp to 0.3.1 and align its @primer/react dependency to ^38.15.0.
• Update the @next (release-candidate) workflow to append -rc.<sha> to packages/mcp/package.json as well.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
package-lock.json	Updates locked workspace/example references for @primer/react@38.15.0 and @primer/mcp@0.3.1 (including MCP’s @primer/react dependency range).
.github/workflows/release.yml	Ensures @primer/mcp gets an -rc.<sha> version suffix during @next publishing, matching existing handling for @primer/react and @primer/styled-react.

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[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/15747

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
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