docs: clarify that provider "main" is for auxiliary tasks only

Users were setting model.provider to "main" after reading the auxiliary
provider docs, causing "Unknown provider" errors. The "main" alias is
only valid inside auxiliary:, compression:, and fallback_model: configs
where it means "use the same provider as my main agent chat."

Added warning admonitions and inline clarifications to:
- configuration.md: Auxiliary Models provider list and Provider Options table
- fallback-providers.md: Provider Options for Auxiliary Tasks table

Reported by community member cn on Discord.

website/docs/user-guide/configuration.md

@@ -553,7 +553,11 @@ Every model slot in Hermes — auxiliary tasks, compression, fallback — uses t
 
 When `base_url` is set, Hermes ignores the provider and calls that endpoint directly (using `api_key` or `OPENAI_API_KEY` for auth). When only `provider` is set, Hermes uses that provider's built-in auth and base URL.
 
-Available providers: `auto`, `openrouter`, `nous`, `codex`, `copilot`, `anthropic`, `main`, `zai`, `kimi-coding`, `minimax`, any provider registered in the [provider registry](/docs/reference/environment-variables), or any named custom provider from your `custom_providers` list (e.g. `provider: "beans"`).
+Available providers for auxiliary tasks: `auto`, `openrouter`, `nous`, `codex`, `copilot`, `anthropic`, `main`, `zai`, `kimi-coding`, `minimax`, any provider registered in the [provider registry](/docs/reference/environment-variables), or any named custom provider from your `custom_providers` list (e.g. `provider: "beans"`).
+
+:::warning `"main"` is for auxiliary tasks only
+The `"main"` provider option means "use whatever provider my main agent uses" — it's only valid inside `auxiliary:`, `compression:`, and `fallback_model:` configs. It is **not** a valid value for your top-level `model.provider` setting. If you use a custom OpenAI-compatible endpoint, set `provider: custom` in your `model:` section. See [AI Providers](/docs/integrations/providers) for all main model provider options.
+:::
 
 ### Full auxiliary config reference
 
@@ -647,13 +651,15 @@ AUXILIARY_VISION_MODEL=openai/gpt-4o
 
 ### Provider Options
 
+These options apply to **auxiliary task configs** (`auxiliary:`, `compression:`, `fallback_model:`), not to your main `model.provider` setting.
+
 | Provider | Description | Requirements |
 |----------|-------------|-------------|
 | `"auto"` | Best available (default). Vision tries OpenRouter → Nous → Codex. | — |
 | `"openrouter"` | Force OpenRouter — routes to any model (Gemini, GPT-4o, Claude, etc.) | `OPENROUTER_API_KEY` |
 | `"nous"` | Force Nous Portal | `hermes auth` |
 | `"codex"` | Force Codex OAuth (ChatGPT account). Supports vision (gpt-5.3-codex). | `hermes model` → Codex |
-| `"main"` | Use your active custom/main endpoint. This can come from `OPENAI_BASE_URL` + `OPENAI_API_KEY` or from a custom endpoint saved via `hermes model` / `config.yaml`. Works with OpenAI, local models, or any OpenAI-compatible API. | Custom endpoint credentials + base URL |
+| `"main"` | Use your active custom/main endpoint. This can come from `OPENAI_BASE_URL` + `OPENAI_API_KEY` or from a custom endpoint saved via `hermes model` / `config.yaml`. Works with OpenAI, local models, or any OpenAI-compatible API. **Auxiliary tasks only — not valid for `model.provider`.** | Custom endpoint credentials + base URL |
 
 ### Common Setups
 

website/docs/user-guide/features/fallback-providers.md

@@ -240,13 +240,15 @@ All three — auxiliary, compression, fallback — work the same way: set `provi
 
 ### Provider Options for Auxiliary Tasks
 
+These options apply to `auxiliary:`, `compression:`, and `fallback_model:` configs only — `"main"` is **not** a valid value for your top-level `model.provider`. For custom endpoints, use `provider: custom` in your `model:` section (see [AI Providers](/docs/integrations/providers)).
+
 | Provider | Description | Requirements |
 |----------|-------------|-------------|
 | `"auto"` | Try providers in order until one works (default) | At least one provider configured |
 | `"openrouter"` | Force OpenRouter | `OPENROUTER_API_KEY` |
 | `"nous"` | Force Nous Portal | `hermes auth` |
 | `"codex"` | Force Codex OAuth | `hermes model` → Codex |
-| `"main"` | Use whatever provider the main agent uses | Active main provider configured |
+| `"main"` | Use whatever provider the main agent uses (auxiliary tasks only) | Active main provider configured |
 | `"anthropic"` | Force Anthropic native | `ANTHROPIC_API_KEY` or Claude Code credentials |
 
 ### Direct Endpoint Override