docs: update all docs for /model command overhaul and custom provider support

Documents the full /model command overhaul across 6 files:

AGENTS.md:
- Add model_switch.py to project structure tree

configuration.md:
- Rewrite General Setup with 3 config methods (interactive, config.yaml, env vars)
- Add new 'Switching Models with /model' section documenting all syntax variants
- Add 'Named Custom Providers' section with config.yaml examples and
  custom:name:model triple syntax

slash-commands.md:
- Update /model descriptions in both CLI and messaging tables with
  full syntax examples (provider:model, custom:model, custom:name:model,
  bare custom auto-detect)

cli-commands.md:
- Add /model slash command subsection under hermes model with syntax table
- Add custom endpoint config to hermes model use cases

faq.md:
- Add config.yaml example for offline/local model setup
- Note that provider: custom is a first-class provider
- Document /model custom auto-detect

provider-runtime.md:
- Add model_switch.py to implementation file list
- Update provider families to show Custom as first-class with named variants

website/docs/reference/slash-commands.md

@@ -40,7 +40,7 @@ Type `/` in the CLI to open the autocomplete menu. Built-in commands are case-in
 | Command | Description |
 |---------|-------------|
 | `/config` | Show current configuration |
-| `/model` | Show or change the current model |
+| `/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) |
 | `/provider` | Show available providers and current provider |
 | `/prompt` | View/set custom system prompt |
 | `/personality` | Set a predefined personality |
@@ -98,7 +98,7 @@ 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, including provider switches. |
+| `/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`). |
 | `/provider` | Show provider availability and auth status. |
 | `/personality [name]` | Set a personality overlay for the session. |
 | `/retry` | Retry the last message. |