ci: publish prerelease builds to npm under the next dist-tag

[zerts]

What

Adds a QA/prerelease channel for the CLI so teammates can try merged-but-unreleased improvements without touching what regular users install.

• New workflow .github/workflows/publish-next.yml — on every push to main (skipping release-please's own chore(main): release commits) it runs tests, bumps to a unique prerelease version like 1.5.1-next.20260714093000.g325093a, and runs npm publish --provenance --tag next. The --tag next flag means the latest dist-tag never moves — only merging the release-please PR promotes a real release.
• workflow_dispatch — a prerelease can also be published manually from any branch via the Actions tab.
• README — documents the @next channel, git-installing unmerged branches (npm i -g github:zeriontech/zerion-ai#<branch>), and how to roll the next tag back to a known-good version with npm dist-tag add.

QA flow this enables

1. Merge PRs to main → each merge publishes to @next
2. Teammates test with npx zerion-cli@next / npm i -g zerion-cli@next
3. When solid, merge the release-please PR → publishes to latest for everyone

Notes

• Uses the existing NPM_TOKEN secret (same package, different dist-tag).
• The prerelease version bumps patch over the current stable so it sorts above it in semver; the g prefix on the SHA avoids invalid all-digit-with-leading-zero prerelease identifiers.
• A concurrency group serializes runs so an older slow build can't move the next tag past a newer one.
• Old -next.* versions accumulate on npm harmlessly; npm forbids republishing versions so no cleanup is needed.

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