refactor: update build config for @primer/mcp to rolldown

[joshblack]

Closes #

N/A

Changelog

New

N/A

Changed

• Updated @primer/mcp to build JavaScript and declarations with Rolldown.

Removed

• Removed the package Rollup build config and Rollup build dependencies.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; build tooling-only change with no public runtime API change

Testing & Reviewing

Validated as part of the stack with npm run build, npm run type-check, npm run lint, npm run lint:css, npm run format:diff, and npm test -- --run.

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)

[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[changeset-bot]

⚠️ No Changeset found

Latest commit: d0d3e65

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[Copilot]

Pull request overview

This PR migrates the @primer/mcp package build tooling from Rollup to Rolldown, aiming to keep the same published JS + type declaration outputs while removing Rollup-specific config and dependencies.

Changes:

• Removed the Rollup build configuration (rollup.config.js).
• Added a Rolldown multi-step build configuration to emit JS and .d.ts outputs (rolldown.config.js).
• Updated packages/mcp/package.json scripts and devDependencies to use Rolldown instead of Rollup.

Show a summary per file

File	Description
packages/mcp/rollup.config.js	Removes the old Rollup-based build pipeline configuration.
packages/mcp/rolldown.config.js	Introduces Rolldown config for JS bundling + separate declaration generation.
packages/mcp/package.json	Switches build/watch scripts to Rolldown and replaces Rollup/Babel build deps with Rolldown deps.

Copilot's findings

• Files reviewed: 3/4 changed files
• Comments generated: 3