ci: switch to release-please for fully automated releases

[abruption]

Replace the manual npm version + conventional-changelog flow with release-please.

Behavior

• On push to main, release-please opens/maintains a release PR (bumps version + CHANGELOG from Conventional Commits).
• Merging that PR creates the GitHub Release + tag, then the same workflow runs npm publish --provenance.
• No more manual npm version && git push --follow-tags.

Changes

• .github/workflows/release-please.yml (manage release PR + publish on release_created)
• remove .github/workflows/release.yml (superseded)
• release-please-config.json + .release-please-manifest.json (seed 0.2.0)
• remove conventional-changelog-cli devDep + version/changelog scripts (release-please owns CHANGELOG)
• enable repo setting: Actions can create PRs
• docs updated