docs: clarify hermes model vs /model — two commands, two purposes (#10276)

Users are confused about the difference between `hermes model` (terminal
command for full provider setup) and `/model` (session command for switching
between already-configured providers). This distinction was not documented
anywhere.

Changes across 4 doc pages:
- cli-commands.md: Added warning callout explaining the difference, added
  --global flag docs, added 'only see OpenRouter models?' info box
- slash-commands.md: Added notes on both TUI and messaging /model entries
  that /model only switches between configured providers
- providers.md: Added 'Two Commands for Model Management' comparison table
  near top of page, added warning callout in switching section
- faq.md: Added new FAQ entry '/model only shows one provider' with quick
  reference table

Prompted by user feedback in Discord — new users consistently hit this
confusion when trying to add providers from inside a session.

website/docs/reference/slash-commands.md

@@ -46,7 +46,7 @@ Type `/` in the CLI to open the autocomplete menu. Built-in commands are case-in
 | Command | Description |
 |---------|-------------|
 | `/config` | Show current configuration |
-| `/model [model-name]` | Show or change the current model. Supports: `/model claude-sonnet-4`, `/model provider:model` (switch providers), `/model custom:model` (custom endpoint), `/model custom:name:model` (named custom provider), `/model custom` (auto-detect from endpoint). Use `--global` to persist the change to config.yaml. |
+| `/model [model-name]` | Show or change the current model. Supports: `/model claude-sonnet-4`, `/model provider:model` (switch providers), `/model custom:model` (custom endpoint), `/model custom:name:model` (named custom provider), `/model custom` (auto-detect from endpoint). Use `--global` to persist the change to config.yaml. **Note:** `/model` can only switch between already-configured providers. To add a new provider, exit the session and run `hermes model` from your terminal. |
 | `/provider` | Show available providers and current provider |
 | `/personality` | Set a predefined personality |
 | `/verbose` | Cycle tool progress display: off → new → all → verbose. Can be [enabled for messaging](#notes) via config. |
@@ -124,7 +124,7 @@ The messaging gateway supports the following built-in commands inside Telegram,
 | `/reset` | Reset conversation history. |
 | `/status` | Show session info. |
 | `/stop` | Kill all running background processes and interrupt the running agent. |
-| `/model [provider:model]` | Show or change the model. Supports provider switches (`/model zai:glm-5`), custom endpoints (`/model custom:model`), named custom providers (`/model custom:local:qwen`), and auto-detect (`/model custom`). Use `--global` to persist the change to config.yaml. |
+| `/model [provider:model]` | Show or change the model. Supports provider switches (`/model zai:glm-5`), custom endpoints (`/model custom:model`), named custom providers (`/model custom:local:qwen`), and auto-detect (`/model custom`). Use `--global` to persist the change to config.yaml. **Note:** `/model` can only switch between already-configured providers. To add a new provider or set up API keys, use `hermes model` from your terminal (outside the chat session). |
 | `/provider` | Show provider availability and auth status. |
 | `/personality [name]` | Set a personality overlay for the session. |
 | `/fast [normal\|fast\|status]` | Toggle fast mode — OpenAI Priority Processing / Anthropic Fast Mode. |