feat(small-model): resolve plugin-registered providers from the running OpenCode

Plugin providers are registered from a plugin's `config` hook and credentialed
from its `auth` loader, both inside the running OpenCode process. Nothing about
them reaches `opencode.json` or `auth.json`, so resolution that only reads files
could not see them: selecting such a model failed with "has no known API base
URL" while the same model worked in chat (#2666).

`GET /provider` is where that state is visible. A new `runtime-providers`
module keeps one cached snapshot of it and reports, per provider, the
credential and endpoint OpenCode itself resolved. Credential resolution becomes
config -> runtime -> auth.json, and endpoint resolution config -> openai default
-> runtime -> models.dev catalog.

Providers with a dedicated wire format (Copilot, ChatGPT-plan OpenAI, Anthropic,
Google) are excluded from the runtime credential: for them OpenCode reports an
OAuth access token that their real transport does not accept.

opencode zen is excluded when the user has no zen login. OpenCode then reports
the sentinel `apiKey: "public"` and trims its catalog to free models that run on
its own infrastructure; the sentinel is never read as a credential.

Claude Code stays refused for background actions even when a plugin publishes an
OpenAI-compatible endpoint for it, because that endpoint is a facade over the
Claude Agent SDK and spawns the CLI per request.

No capability probe. Asking `GET /models` does identify a plugin whose protocol
lives in its own `fetch`, but measured across the 166 providers with an `api`
URL in the models.dev catalog it also denies six that work and simply have no
`/models` route. A provider that vanishes from the picker explains nothing,
while one that fails on use says why, so availability stops at credential and
endpoint.

The same list drives the Small Model and Changes Walkthrough pickers.

Validated against a real OpenCode with four plugin providers loaded: offered
providers went from 3 to 7, zen and Claude Code stayed out, and a generation
through a plugin-backed model that previously failed now returns.

packages/web/server/index.js

@@ -72,6 +72,7 @@ import { createOpenCodeResolutionRuntime } from './lib/opencode/opencode-resolut
 import { resolveOpenCodeUpgradeCapability } from './lib/opencode/upgrade-capability.js';
 import { createBootstrapRuntime } from './lib/opencode/bootstrap-runtime.js';
 import { createSessionRuntime } from './lib/opencode/session-runtime.js';
+import { configureOpenCodeRuntimeProviders, resetOpenCodeRuntimeProviders } from './lib/small-model/runtime-providers.js';
 import { createOpenCodeWatcherRuntime } from './lib/opencode/watcher.js';
 import { createSessionAssistRuntime } from './lib/session-assist/runtime.js';
 import { createSessionGoalRuntime } from './lib/session-goal/runtime.js';
@@ -664,6 +665,11 @@ const buildOpenCodeUrl = (...args) => openCodeNetworkRuntime.buildOpenCodeUrl(..
 const ensureOpenCodeApiPrefix = (...args) => openCodeNetworkRuntime.ensureOpenCodeApiPrefix(...args);
 const scheduleOpenCodeApiDetection = (...args) => openCodeNetworkRuntime.scheduleOpenCodeApiDetection(...args);
 
+// Plugin-registered providers exist only inside the running OpenCode process.
+// Small-model callers resolve them through this connection; without it they
+// stay on the file-based resolution and plugin models remain unreachable.
+configureOpenCodeRuntimeProviders({ buildOpenCodeUrl, getOpenCodeAuthHeaders });
+
 const ENV_CONFIGURED_API_PREFIX = normalizeApiPrefix(
   process.env.OPENCODE_API_PREFIX || process.env.OPENCHAMBER_API_PREFIX || ''
 );
@@ -1162,6 +1168,9 @@ const openCodeLifecycleRuntime = createOpenCodeLifecycleRuntime({
   // process (#2638). The runtime is created later by the startup pipeline;
   // by the time any restart runs, it is assigned.
   onOpenCodeRestarted: () => {
+    // A restart reloads plugins: provider ports, credentials and the provider
+    // list itself can all differ from what was cached.
+    resetOpenCodeRuntimeProviders();
     try {
       messageStreamRuntime?.rebindUpstream();
     } catch (error) {