docs: clarify Token Plan vs pay-per-token billing (key + model matrix)

Field investigation against the live API:
- sk-cp- (Token Plan)  + MiniMax-M2.7           → ✅ works
- sk-cp- (Token Plan)  + MiniMax-M2.7-highspeed → ❌ "Token Plan Starter
  (0/0 used)" — the base Token Plan EXCLUDES the highspeed variant
- sk-api- (pay-per-token) + either model        → ✅ structurally
  (needs balance > 0; otherwise "insufficient_balance (1008)")

Plugin code already handles both key types via the same
Authorization: Bearer header — no functional change needed. README now
makes the matrix explicit so operators don't try highspeed on a
plan-only key and wonder why "0/0 used" comes back.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -12,6 +12,22 @@ Anthropic-compatible HTTP endpoint.
 **Deferred**: OAuth (Code Plan portal), MiniMax Portal provider, image /
 TTS / video / music providers (separate plugins later if wanted).
 
+## API key + model matrix
+
+MiniMax issues two key types from the same Anthropic-compatible endpoint;
+this plugin sends both with `Authorization: Bearer <key>` and lets the
+server decide. The key prefix tells you which billing model you're on:
+
+| key prefix | billing | `MiniMax-M2.7` | `MiniMax-M2.7-highspeed` |
+|---|---|---|---|
+| `sk-cp-...` | **Token Plan** (per-plan quota) | ✅ included | ❌ `Token Plan Starter (0/0 used)` — highspeed is NOT in the base Token Plan |
+| `sk-api-...` | **pay-per-token** (per-request billed) | ✅ if balance > 0; else `insufficient_balance (1008)` | ✅ if balance > 0 |
+
+**Takeaway**: if you only have a Token Plan key, set the agent's model
+to `MiniMax-M2.7` (base). For the highspeed variant you need a funded
+`sk-api-` key. The plugin doesn't reject highspeed at startup — the
+error surfaces in the agent's response when the model is invoked.
+
 ## Install
 
 ```bash