OpenRouter models never appear in model picker (supported-models ACP endpoint returns empty list)

### Before filing

- [x] I searched open and closed issues for duplicates.
- [x] I reproduced this on the latest release.
- [x] This is one bug, not several bundled together.

### Closest existing issue

#154 (closed, fixed in #163). While #154 addressed preventing an empty refresh from wiping existing cache entries, it did not address why OpenRouter returns an empty list in the first place.

### What's broken

Models for the OpenRouter provider never appear in Berd's model picker, even when OpenRouter is configured with a valid API key and active in Goose.

Berd populates the model picker via `fetchProviderSupportedModels` (`src/features/providers/stores/providerModelCacheStore.ts`), calling the backend ACP method `_goose/unstable/providers/supported-models/list` (`GooseUnstableProvidersSupportedModelsList`). In `goosed` (`crates/goose/src/acp/server/providers.rs`), this ACP call delegates to `provider.fetch_supported_models()`.

However, in `aaif-goose/goose`:
- `crates/goose-provider-types/src/base.rs` provides a default implementation for `fetch_supported_models` that returns `Ok(vec![])`.
- `crates/goose-providers/src/openrouter.rs` implements `fetch_recommended_models` (which calls OpenRouter's `/api/v1/models` and filters for tool support), but **does not implement `fetch_supported_models`**.

As a result, `_goose/unstable/providers/supported-models/list` for `openrouter` always returns `{"models": []}`. Because no models are ever returned, the model picker remains completely empty for OpenRouter.

### Steps to reproduce

1. Launch Berd.
2. Configure OpenRouter as the provider for Goose with a valid API key and a model (e.g. `google/gemini-3.8-flash` or `anthropic/claude-sonnet-4.5`).
3. Open a chat and verify that Goose functions with OpenRouter.
4. Open the model picker for Goose in Berd.
5. Search or browse for OpenRouter models.

### What you expected to happen

OpenRouter models supported by Goose (or returned by OpenRouter) appear in the model picker.

### What actually happened

Zero OpenRouter models are listed in the picker. Inspecting `localStorage` (`goose:providerModelCache:v1`) confirms `openrouter` has an empty models array: `{"providerId":"openrouter","models":[],"fetchedAt":...}`. Direct ACP queries to `_goose/unstable/providers/supported-models/list` with `{"providerId": "openrouter"}` also return `{"models": []}`.

### How often does it happen?

Every time — reliably reproducible

### Berd version

0.6.2 (and verified upstream code against latest 0.6.3 / main)

### Operating system

macOS (Apple Silicon)

### Model and provider

Goose agent, OpenRouter provider (e.g. `google/gemini-3.8-flash` or any OpenRouter model)

### Relevant log output

Direct query to bundled `goosed` via ACP:
```json
Request:
{"jsonrpc":"2.0","id":1,"method":"_goose/unstable/providers/supported-models/list","params":{"providerId":"openrouter"}}

Response:
{"jsonrpc":"2.0","id":1,"result":{"models":[],"providerId":"openrouter"}}
```

Local storage cache entry in Berd's WebKit database (`localstorage.sqlite3`):
```json
{"providerId":"openrouter","models":[],"fetchedAt":1788805485945}
```

### Screenshots, recordings, or other context

Upstream code pointers in `aaif-goose/goose`:
1. `crates/goose-providers/src/openrouter.rs`: implements `fetch_recommended_models` using OpenRouter `/api/v1/models`, but does not implement `fetch_supported_models`.
2. `crates/goose-provider-types/src/base.rs`:
```rust
async fn fetch_supported_models(&self) -> Result, ProviderError> {
Ok(vec![])
}
```
3. `crates/goose/src/acp/server/providers.rs`:
```rust
pub(super) async fn on_list_provider_supported_models(
&self,
req: ProviderSupportedModelsListRequest,
) -> Result {
let provider = self
.create_provider(&req.provider_id, Vec::new(), None, true)
.await
.internal_err_ctx("Failed to initialize provider")?;
let models = match provider.fetch_supported_models().await {
...
```

Start by reproducing the empty response with the `_goose/unstable/providers/supported-models/list` ACP request. Read `src/features/providers/stores/providerModelCacheStore.ts`, `crates/goose-providers/src/openrouter.rs`, `crates/goose-provider-types/src/base.rs`, and `crates/goose/src/acp/server/providers.rs` to trace the model flow. Done means OpenRouter models returned by the endpoint appear in Berd’s model picker.

Written by the indexing model from the issue text.