docs(env): document Z.ai GLM API + devpack CLI routing and base-URL override

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -42,8 +42,14 @@ MISTRAL_API_KEY=
 # qwen:        https://dashscope-intl.aliyuncs.com/  (Alibaba DashScope)
 DASHSCOPE_API_KEY=
 
-# zai:         https://z.ai/  (GLM models)
+# zai:         https://z.ai/  (GLM 5.1 / 5.2 / 5.3). Works as a plain API and,
+#              with --subscription, drives GLM through the Claude Code CLI via
+#              Z.ai's Anthropic-compatible endpoint (the devpack / cline / zcode
+#              setup — https://docs.z.ai/devpack/tool/cline).
 ZAI_API_KEY=
+# Optional: override Z.ai's Anthropic-compatible base URL (default
+# https://api.z.ai/api/anthropic) for a self-hosted gateway.
+# ZAI_ANTHROPIC_BASE_URL=
 
 # moonshot:    https://platform.moonshot.ai/  (Kimi models)
 MOONSHOT_API_KEY=