ci: add release-please and Docker image publishing pipeline

[jordangarrison]

Summary

• Add release-please automation for semantic versioning, changelog generation, and GitHub Releases
• Add publish workflow that pushes Docker images tagged latest + vX.Y.Z on each release
• Change CI deploy job to tag main branch images as edge instead of latest
• Add PR title validation to enforce conventional commit format
• Add x-release-please-version annotations to mix.exs and nix/package.nix for automatic version bumping

Docker Image Tagging Strategy

Trigger	Tags	Purpose
Push to main	edge, <sha>	Bleeding edge builds
Release published	latest, vX.Y.Z	Stable releases

New Workflows

• release-please.yml — creates release PRs with changelog on push to main
• publish.yml — builds and pushes versioned Docker images on GitHub Release
• check-pr-title.yml — validates PR titles follow conventional commits

Test Plan

• CI passes on this PR
• After merge, release-please creates a release PR
• Merging the release PR creates a GitHub Release with v0.2.0 tag
• Publish workflow triggers and pushes latest + v0.2.0 to GHCR
• Main branch pushes tag Docker images as edge