chore: Enter alpha prerelease mode

[DouweBos]

What

Re-enters changesets pre mode with tag alpha by re-adding .changeset/pre.json. This was previously removed in 12e105e ("Remove prerelease") when the repo exited prerelease.

Why

We want to start shipping alpha versions again. Pre mode's presence is driven entirely by pre.json — while it exists on main, changeset version/publish (via the release workflow) produce -alpha.N versions on the npm alpha dist-tag instead of stable bumps on latest.

After merge

• The release workflow's "Version Packages" PR will target alpha versions.
• Continue adding changesets per change as usual; they're consumed as alpha bumps while pre mode is active.
• To go stable again, run changeset pre exit and remove pre.json.