feat(xiaomi): add Xiaomi MiMo as first-class provider

Cherry-picked from PR #7702 by kshitijk4poor.

Adds Xiaomi MiMo as a direct provider (XIAOMI_API_KEY) with models:
- mimo-v2-pro (1M context), mimo-v2-omni (256K, multimodal), mimo-v2-flash (256K, cheapest)

Standard OpenAI-compatible provider checklist: auth.py, config.py, models.py,
main.py, providers.py, doctor.py, model_normalize.py, model_metadata.py,
models_dev.py, auxiliary_client.py, .env.example, cli-config.yaml.example.

Follow-up: vision tasks use mimo-v2-omni (multimodal) instead of the user's
main model. Non-vision aux uses the user's selected model. Added
_PROVIDER_VISION_MODELS dict for provider-specific vision model overrides.
On failure, falls back to aggregators (gemini flash) via existing fallback chain.

Corrects pre-existing context lengths: mimo-v2-pro 1048576→1000000,
mimo-v2-omni 1048576→256000, adds mimo-v2-flash 256000.

36 tests covering registry, aliases, auto-detect, credentials, models.dev,
normalization, URL mapping, providers module, doctor, aux client, vision
model override, and agent init.

hermes_cli/models.py

@@ -188,6 +188,11 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
         "deepseek-chat",
         "deepseek-reasoner",
     ],
+    "xiaomi": [
+        "mimo-v2-pro",
+        "mimo-v2-omni",
+        "mimo-v2-flash",
+    ],
     "opencode-zen": [
         "gpt-5.4-pro",
         "gpt-5.4",
@@ -493,6 +498,7 @@ _PROVIDER_LABELS = {
     "alibaba": "Alibaba Cloud (DashScope)",
     "qwen-oauth": "Qwen OAuth (Portal)",
     "huggingface": "Hugging Face",
+    "xiaomi": "Xiaomi MiMo",
     "custom": "Custom endpoint",
 }
 
@@ -535,6 +541,8 @@ _PROVIDER_ALIASES = {
     "hf": "huggingface",
     "hugging-face": "huggingface",
     "huggingface-hub": "huggingface",
+    "mimo": "xiaomi",
+    "xiaomi-mimo": "xiaomi",
 }
 
 
@@ -819,7 +827,7 @@ def list_available_providers() -> list[dict[str, str]]:
         "openrouter", "nous", "openai-codex", "copilot", "copilot-acp",
         "gemini", "huggingface",
         "zai", "kimi-coding", "minimax", "minimax-cn", "kilocode", "anthropic", "alibaba",
-        "qwen-oauth",
+        "qwen-oauth", "xiaomi",
         "opencode-zen", "opencode-go",
         "ai-gateway", "deepseek", "custom",
     ]