fix: complete MiniMax image provider contracts

docs/PROVIDERS.md

@@ -50,6 +50,7 @@ AZURE_SPEECH_REGION=         # Speech resource region, e.g. eastus
 
 # MULTI-MODEL GATEWAY (one key, 6+ tools)
 FAL_KEY=                     # FLUX, Recraft, Kling, Veo, MiniMax video
+MINIMAX_API_KEY=             # MiniMax first-party image generation
 
 # KLING OFFICIAL DIRECT API
 KLING_API_KEY=               # Official Kling video, image, TTS, avatar, lip sync
@@ -227,6 +228,40 @@ No subscription — pure pay-as-you-go, no minimum spend.
 
 ---
 
+### MiniMax — Official Direct Image API
+
+> **Low-cost first-party image generation.** The direct MiniMax API supports
+> seeded text-to-image, character subject references, custom dimensions, and
+> global or mainland-China routing without a gateway.
+
+**Tool unlocked:** `minimax_image`
+
+**Env var:** `MINIMAX_API_KEY`
+
+**Optional region:** `MINIMAX_REGION=global` (default) or `cn`
+
+#### Setup
+
+1. Create a MiniMax Open Platform account.
+2. Generate an API key in the account's API-key page.
+3. Add `MINIMAX_API_KEY=...` to `.env`.
+4. For a mainland-China account, also set `MINIMAX_REGION=cn`.
+
+#### Pricing
+
+| Models | Global pay-as-you-go price |
+|--------|----------------------------|
+| `image-01`, `image-01-live` | $0.0035 per generated image |
+
+MiniMax also offers subscription token plans with included daily image quota.
+OpenMontage conservatively reports the standard pay-as-you-go amount in cost
+estimates and generation results.
+
+The tool is automatically discoverable through `image_selector`; choose it
+with `preferred_provider: "minimax"`.
+
+---
+
 ### Kling Official — Direct API
 
 > **Official Kling path.** This is separate from `kling_video` via fal.ai: it uses Kling's official `Authorization: Bearer <KLING_API_KEY>` API, provider name `kling_official`, and direct Classic/Turbo/Omni task protocols.