feat(providers): split zai api_key_env from zhipu (#84)

`zai` (api.z.ai, the international front-end) and `zhipu` (open.bigmodel.cn,
the China front-end) target the same Zhipu account system but are surfaced
as distinct providers in the dashboard. They previously both declared
`api_key_env = "ZHIPU_API_KEY"`, so configuring a single Zhipu credential
silently activated both — same root cause as the cross-product collisions
fixed in librefang-registry#82 and librefang-registry#83.

Rename `zai` to use its own `ZAI_API_KEY`. `zhipu` keeps `ZHIPU_API_KEY`
as the more-established name.

This is a breaking change: existing users authenticating `zai` via
`ZHIPU_API_KEY` need to also export `ZAI_API_KEY` (same key value works).
The companion daemon-side change will be filed against librefang/librefang.

Refs librefang/librefang#3282

providers/zai.toml

@@ -1,10 +1,16 @@
-# Z.AI — https://api.z.ai (Zhipu domain alias)
+# Z.AI — https://api.z.ai (international front-end of the same Zhipu backend)
 # Models: 2
+#
+# `zai` and `zhipu` (open.bigmodel.cn) point at the same upstream account
+# system but are surfaced as distinct providers in the dashboard. Each now
+# uses its own env so configuring one key doesn't auto-activate both —
+# export the same Zhipu API key value under whichever name(s) you actually
+# use. See librefang/librefang#3282.
 
 [provider]
 id = "zai"
 display_name = "Z.AI"
-api_key_env = "ZHIPU_API_KEY"
+api_key_env = "ZAI_API_KEY"
 base_url = "https://api.z.ai/api/paas/v4"
 key_required = true