docs: clarify that provider "main" is for auxiliary tasks only (#6291)

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/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