docs(releaser): release v1.5.1 — documentation site, fuzzing completeness

- Add Hugo + Geekdoc documentation site (docs/): installation, CLI
  reference, configuration, CI integration, and changelog pages; explicit
  menu bundle nav so all pages appear in the sidebar on every page
- Add Gitea CI workflow (.gitea/workflows/docs.yml): builds on push to
  main when docs/** changes, deploys minified site to gh-pages branch
- Add docs:setup / docs:serve / docs:build Taskfile tasks; theme bundle
  downloaded at build time (not committed)
- Add FuzzUpdate to internal/changelog and FuzzWriteVersion to
  internal/node to complete fuzz coverage for all file-rewriting packages
- Add fuzzing completeness guidelines to CLAUDE.md: authoritative table,
  exempt-package rationale, seed corpus rules

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

internal/changelog/changelog_test.go

@@ -149,3 +149,22 @@ func TestUpdateIdempotent(t *testing.T) {
 		t.Error("version heading should appear exactly once after idempotent call")
 	}
 }
+
+// FuzzUpdate verifies Update never panics on arbitrary existing file content or commit messages.
+func FuzzUpdate(f *testing.F) {
+	f.Add("", "feat: add thing")
+	f.Add("# Changelog\n\n## [1.0.0] - 2026-01-01\n\n### Added\n- something\n", "fix: something")
+	f.Add("some preamble\n", "feat!: breaking change")
+	f.Add("\n## [2.0.0] - 2026-01-01\n", "feat: another thing")
+	f.Add("", "chore: no release")
+	f.Add("", "")
+
+	f.Fuzz(func(t *testing.T, existing, message string) {
+		dir := t.TempDir()
+		path := filepath.Join(dir, "CHANGELOG.md")
+		if existing != "" {
+			os.WriteFile(path, []byte(existing), 0644) //nolint:errcheck
+		}
+		Update(path, "v1.0.0", "1.0.0", []string{message}) //nolint:errcheck
+	})
+}