feat: add custom/other OpenAI-compatible LLM providers

Allow Settings → Providers to define custom providers (id, name, base URL,
API key, models, headers) without code changes. Persist config via OpenCode
layers, store keys through auth.set, and keep web/VS Code parity.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/docs/content/docs/providers.mdx

@@ -10,11 +10,20 @@ Before OpenChamber can do anything, it needs at least one AI provider connected.
 ## Connect a provider
 
 1. Open **Settings → Providers**.
-2. Open the **Add provider** menu and pick a provider that isn't connected yet.
+2. Open the **Add provider** menu and pick a provider that isn't connected yet, or choose **Other / Custom** for an OpenAI-compatible endpoint.
 3. Sign in one of two ways, depending on the provider:
    - **API key** — paste your key and save.
    - **Sign-in (device flow)** — OpenChamber shows a link and a short code. Open the link, enter the code, and approve. OpenChamber finishes connecting on its own.
 
+### Custom / Other providers
+
+For gateways, campus LLMs, Ollama, LiteLLM, and similar OpenAI-compatible APIs:
+
+1. Choose **Other / Custom** in the provider list.
+2. Enter a provider ID, display name, base URL (`http://` or `https://`), API key (or `{env:VAR_NAME}`), and at least one model id/name.
+3. Optionally add request headers.
+4. Save — OpenChamber writes the provider block to OpenCode config and stores the key in OpenCode auth.
+
 When a provider shows as connected, its models become available in chat.
 
 To disconnect, open the provider and choose to remove its sign-in.