docs: fix api-server response storage — SQLite, not in-memory (#2819)

* 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

* docs: fix api-server response storage description — SQLite, not in-memory

The ResponseStore class uses SQLite persistence (with in-memory
fallback), not pure in-memory storage. Responses survive gateway
restarts.

website/docs/user-guide/features/api-server.md

@@ -231,6 +231,6 @@ Any frontend that supports the OpenAI API format works. Tested/documented integr
 
 ## Limitations
 
-- **Response storage is in-memory** — stored responses (for `previous_response_id`) are lost on gateway restart. Max 100 stored responses (LRU eviction).
+- **Response storage** — stored responses (for `previous_response_id`) are persisted in SQLite and survive gateway restarts. Max 100 stored responses (LRU eviction).
 - **No file upload** — vision/document analysis via uploaded files is not yet supported through the API.
 - **Model field is cosmetic** — the `model` field in requests is accepted but the actual LLM model used is configured server-side in config.yaml.