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/commits/commits_test.go

@@ -35,6 +35,56 @@ func FuzzParse(f *testing.F) {
 	})
 }
 
+func TestExtractSubject(t *testing.T) {
+	cases := []struct {
+		header string
+		want   string
+	}{
+		{"feat: add login", "add login"},
+		{"feat(auth): add OAuth2", "add OAuth2"},
+		{"feat!: remove API", "remove API"},
+		{"FIX:typo", "typo"},
+		{"plain message", "plain message"},
+	}
+	for _, c := range cases {
+		got := ExtractSubject(c.header)
+		if got != c.want {
+			t.Errorf("ExtractSubject(%q) = %q, want %q", c.header, got, c.want)
+		}
+	}
+}
+
+func TestGroup(t *testing.T) {
+	messages := []string{
+		"feat: add login",
+		"fix: patch null pointer",
+		"feat!: remove legacy API",
+		"chore: update deps",
+		"fix: handle empty response",
+	}
+	breaking, feats, fixes := Group(messages)
+	if len(breaking) != 1 || breaking[0] != "remove legacy API" {
+		t.Errorf("breaking = %v, want [remove legacy API]", breaking)
+	}
+	if len(feats) != 1 || feats[0] != "add login" {
+		t.Errorf("feats = %v, want [add login]", feats)
+	}
+	if len(fixes) != 2 {
+		t.Errorf("fixes = %v, want 2 items", fixes)
+	}
+}
+
+func TestReleasableSet(t *testing.T) {
+	all := ReleasableSet(nil)
+	if !all[TypeFix] || !all[TypeFeat] || !all[TypeBreaking] {
+		t.Error("nil input should return all three types")
+	}
+	only := ReleasableSet([]string{"fix"})
+	if !only[TypeFix] || only[TypeFeat] || only[TypeBreaking] {
+		t.Errorf("fix-only set: %v", only)
+	}
+}
+
 func TestTypeString(t *testing.T) {
 	cases := []struct {
 		t    Type