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/notes/notes_test.go

@@ -69,25 +69,6 @@ func TestGenerateEmpty(t *testing.T) {
 	}
 }
 
-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)
-		}
-	}
-}
-
 // FuzzGenerate verifies that Generate never panics on arbitrary inputs and
 // always includes the tag name in the output.
 func FuzzGenerate(f *testing.F) {