Add configurable Vite runner support

[mdulghier]

Summary

• Added dev_server.runner to choose between vite-plus and vite for devtree dev.
• Kept vite-plus as the default for backward compatibility, and now fail fast on invalid runner values.
• Updated doctor checks, docs, and plugin typing to reflect the new dual-runner setup.

Testing

• Added unit coverage for both runner modes in the command builder.
• Added config command coverage for reading and writing dev_server.runner.
• pnpm test passed.
• pnpm run check passed.
• pnpm run build passed.