ci: add release pipeline with release-please and GoReleaser

Automate versioning and multi-platform binary releases.

## Steps
- Configure release-please for conventional commits
- Configure GoReleaser for cross-platform builds

## Platforms
- linux/amd64
- linux/arm64
- darwin/amd64
- darwin/arm64
- windows/amd64

## Acceptance criteria
- [ ] Merging a `feat:` commit triggers a release PR
- [ ] Merging the release PR publishes binaries to GitHub Releases