feat(releaser): write release.env dotenv artifact on every release

After the next version is determined and dry-run is confirmed off,
release.env is written to the repository root:

  NEXT_VERSION=<nextTag>

This file is not committed — it is left as an untracked artifact so
GitLab CI can expose it as a dotenv artifact and pass NEXT_VERSION to
downstream jobs (e.g. deploy, notify).

The file is skipped in --dry-run mode. Two tests added:
TestRunReleaseEnv and TestRunReleaseEnvDryRun.

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

cmd/main.go

@@ -387,6 +387,13 @@ func run(o options) error {
 		return nil
 	}
 
+	// --- release.env (GitLab CI dotenv artifact) ---
+	releaseEnvPath := filepath.Join(absRepo, "release.env")
+	if err := os.WriteFile(releaseEnvPath, []byte("NEXT_VERSION="+nextTag+"\n"), 0644); err != nil {
+		return fmt.Errorf("write release.env: %w", err)
+	}
+	logDone("release.env: NEXT_VERSION=%s", nextTag)
+
 	// --- pom.xml + CHANGELOG.md (skipped with --tag-only) ---
 	if !o.tagOnly {
 		var filesToCommit []string