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/faq.md

@@ -53,7 +53,16 @@ hermes model
 # Context length: 32768   ← set this to match your server's actual context window
 ```
 
-Hermes persists the endpoint in `config.yaml` and prompts for the context window size so compression triggers at the right time. If you leave context length blank, Hermes auto-detects it from the server's `/models` endpoint or [models.dev](https://models.dev).
+Or configure it directly in `config.yaml`:
+
+```yaml
+model:
+  default: qwen3.5:27b
+  provider: custom
+  base_url: http://localhost:11434/v1
+```
+
+Hermes persists the endpoint, provider, and base URL in `config.yaml` so it survives restarts. If your local server has exactly one model loaded, `/model custom` auto-detects it. You can also set `provider: custom` in config.yaml — it's a first-class provider, not an alias for anything else.
 
 This works with Ollama, vLLM, llama.cpp server, SGLang, LocalAI, and others. See the [Configuration guide](../user-guide/configuration.md) for details.