feat: automate releases from merged PRs

[MehmetSecgin]

Summary

• replace the manual tag-changelog path with semantic-release on merged branches
• stop requiring version bumps in PRs for release work
• document the new commit-driven release behavior and beta path

Notes

• supersedes the manual, tag-based release direction described in Add CI verification and tag-based npm release workflow #3

[MehmetSecgin]

Superseded by split PRs for clearer review:\n- #32 for the dispatch run feature\n- #33 for semantic-release automation