feat(releaser): release v1.4.0 — GitHub, SSH agent, configurable bumps

- GitHub release support: internal/ghclient (no SDK, minimal HTTP client);
  GITHUB_TOKEN env var; github.token/github.repo config; GitHub takes
  precedence over GitLab when both are configured; publisher interface
  (releasePublisher) in cmd/main.go makes providers interchangeable
- SSH agent push: gitutil.Push() now tries gitssh.NewSSHAgentAuth for
  git@/ssh:// remotes before falling back to the system git binary
- --release-env-file flag: override dotenv artifact path; pass "" to disable
- git.releasable_types config: filter which commit types trigger a bump
  (defaults to fix, feat, breaking); useful for maintenance branches
- commits.Group(), ExtractSubject(), ReleasableSet() exported helpers:
  notes.go and changelog.go now delegate to these instead of duplicating
- CHANGELOG deduplication guard: changelog.Update() is idempotent — skips
  write if ## [version] section already exists
- Always load config sources: LoadWithSources() called unconditionally;
  removes the dual config path that previously only tracked sources in
  --verbose mode
- .releaser.gitlab-ci.yml: adds artifacts: reports: dotenv: release.env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ROADMAP.md

@@ -71,11 +71,13 @@
 - [x] ~~Name and version header on every run~~ — ✓ shipped v1.2.0
 - [x] ~~Default tag prefix changed to empty~~ — ✓ shipped v1.2.0 (bare `1.2.3` tags by default; opt in to `v` prefix via config)
 - [x] ~~`release.env` dotenv artifact~~ — ✓ shipped v1.3.0 (`NEXT_VERSION=<tag>` written on every release for GitLab CI downstream jobs)
+- [x] ~~GitHub release support~~ — ✓ shipped v1.4.0 (`internal/ghclient`, `GITHUB_TOKEN` env, `github.token`/`github.repo` config; GitHub takes precedence over GitLab)
+- [x] ~~SSH agent push~~ — ✓ shipped v1.4.0 (go-git `gitssh.NewSSHAgentAuth` for `git@`/`ssh://` remotes)
+- [x] ~~Configurable bump rules~~ — ✓ shipped v1.4.0 (`git.releasable_types` config; filter which commit types trigger a release)
 - [ ] Documentation site
 
 ## Future / backlog
 
-- GitHub release support (parity with GitLab)
 - Multi-module Maven support (multiple `pom.xml` paths)
 - Gradle support (`build.gradle` / `build.gradle.kts`)
 - `package.json` version bump support (Node.js projects)