Prepare 1.0.0-next.0 release

[birkskyum]

Summary

• enter Changesets prerelease mode with the next tag
• make the initial release a major version, producing 1.0.0-next.0
• enable changelog generation for the release action
• update installation guidance to use @next

Context

The first Release run reached the Changesets version step but failed because changelog generation was disabled and CHANGELOG.md did not exist. This enables the standard Changesets changelog and prepares the initial prerelease.

Failed run: https://github.com/solidjs/start-devtools/actions/runs/32290269680

Testing

• pnpm changeset status --output ... reports 1.0.0-next.0
• isolated pnpm changeset version generates CHANGELOG.md with 1.0.0-next.0
• pnpm build
• pnpm test
• pnpm check
• Prettier check