Add develop branch npm prerelease publishing

[Yukaii]

Summary

• Extend the publish workflow to run beta prereleases on pushes to develop
• Keep stable releases on v* tags publishing to npm latest
• Create GitHub pre-releases for each develop build, matching the @hackmd/api pattern

Test plan

• Merge and verify the Pre-release (develop) job publishes to npm with the beta tag
• Confirm install works: npm install -g @hackmd/hackmd-cli@beta
• Confirm stable tag releases still publish to latest unchanged

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