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>

internal/version/version.go

@@ -8,10 +8,14 @@ import (
 
 // Next computes the next version string (without tag prefix, e.g. "1.2.4").
 // currentPatch is -1 when no tag exists yet (first release will be X.Y.0).
+// releasable is the set of commit types that trigger a bump; nil defaults to all three.
 // Returns ("", false) when there are no releasable commits.
-func Next(major, minor, currentPatch int, types []commits.Type) (string, bool) {
+func Next(major, minor, currentPatch int, types []commits.Type, releasable map[commits.Type]bool) (string, bool) {
+	if releasable == nil {
+		releasable = commits.ReleasableSet(nil)
+	}
 	for _, t := range types {
-		if t != commits.TypeNone {
+		if releasable[t] {
 			return fmt.Sprintf("%d.%d.%d", major, minor, currentPatch+1), true
 		}
 	}