feat(ai): add Xiaomi MiMo provider (#4005)

* fix(ai): include minimax-cn in cross-provider-handoff matrix

* feat(ai): add Xiaomi MiMo provider

Adds Xiaomi MiMo as an openai-completions-compatible provider.

- packages/ai: register provider in types/KnownProvider, env-api-keys (XIAOMI_API_KEY), generate-models, models.generated.ts, overflow util, README, CHANGELOG
- packages/ai/test: extend stream, tokens, abort, empty, context-overflow, overflow, image-tool-result, tool-call-without-result, total-tokens, unicode-surrogate, cross-provider-handoff matrices with Xiaomi
- packages/coding-agent: default model (mimo-v2.5-pro), display name (Xiaomi MiMo), CLI env var docs, README, docs/providers.md

closes #3912

---------

Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

packages/coding-agent/docs/providers.md

@@ -69,6 +69,7 @@ pi
 | Kimi For Coding | `KIMI_API_KEY` | `kimi-coding` |
 | MiniMax | `MINIMAX_API_KEY` | `minimax` |
 | MiniMax (China) | `MINIMAX_CN_API_KEY` | `minimax-cn` |
+| Xiaomi MiMo | `XIAOMI_API_KEY` | `xiaomi` |
 
 Reference for environment variables and `auth.json` keys: [`const envMap`](https://github.com/badlogic/pi-mono/blob/main/packages/ai/src/env-api-keys.ts) in [`packages/ai/src/env-api-keys.ts`](https://github.com/badlogic/pi-mono/blob/main/packages/ai/src/env-api-keys.ts).
 
@@ -83,7 +84,8 @@ Store credentials in `~/.pi/agent/auth.json`:
   "deepseek": { "type": "api_key", "key": "sk-..." },
   "google": { "type": "api_key", "key": "..." },
   "opencode": { "type": "api_key", "key": "..." },
-  "opencode-go": { "type": "api_key", "key": "..." }
+  "opencode-go": { "type": "api_key", "key": "..." },
+  "xiaomi": { "type": "api_key", "key": "..." }
 }
 ```