ci: integrate release-please

[khvn26]

Summary

• Add release-please workflow + config (mirrors flagsmith-nodejs-client setup) to automate version bumps and CHANGELOG generation from Conventional Commits on main.
• Seed .release-please-manifest.json at the current package.json version (1.0.0).
• Add .github/dependabot.yml with deps commit prefix (for both prod and dev) so Dependabot PRs land under release-please's "Dependency Updates" section instead of "Other".

Test plan

• After merge, confirm release-please opens a release PR on the next push to main.
• Confirm next Dependabot PR uses deps: / deps(deps-dev): prefix.
• Confirm existing publish-package.yml still runs on the GitHub release published by release-please.

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review to trigger a review and subscribe this PR to future pushes, or @claude review once for a one-time review.

_{Tip: disable this comment in your organization's Code Review settings.}