refactor(v2/providers): self-registration barrel + host container-config registry

Providers now mirror the channels pattern: each module calls
registerProvider() at top level, and providers/index.ts is a barrel of
side-effect imports. createProvider() becomes a thin registry lookup;
the closed ProviderName union is gone (now a string alias, since the
env var is a runtime string anyway).

Also adds a host-side provider-container-registry so providers can
declare their own mounts and env passthrough in src/providers/<name>.ts
instead of the container-runner having to know about each one. The
resolver runs once per spawn and threads provider + contribution
through buildMounts and buildContainerArgs so side effects (mkdir,
etc.) fire exactly once.

Both barrels are append-only — adding a new provider is a new file
+ one import line per barrel, no edits to existing files. The built-in
providers (claude, mock) don't need host-side config, so src/providers/
ships with an empty barrel; the container-side barrel imports both.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/agent-runner/src/providers/provider-registry.ts

@@ -0,0 +1,33 @@
+/**
+ * Provider self-registration registry.
+ *
+ * Mirrors `src/channels/channel-registry.ts` on the host. Each provider module
+ * calls `registerProvider()` at top level; the barrel (`providers/index.ts`)
+ * imports every provider module for its side effect so registrations fire
+ * before `createProvider()` is called.
+ */
+import type { AgentProvider, ProviderOptions } from './types.js';
+
+export type ProviderFactory = (options: ProviderOptions) => AgentProvider;
+
+const registry = new Map<string, ProviderFactory>();
+
+export function registerProvider(name: string, factory: ProviderFactory): void {
+  if (registry.has(name)) {
+    throw new Error(`Provider already registered: ${name}`);
+  }
+  registry.set(name, factory);
+}
+
+export function getProviderFactory(name: string): ProviderFactory {
+  const factory = registry.get(name);
+  if (!factory) {
+    const known = [...registry.keys()].join(', ') || '(none)';
+    throw new Error(`Unknown provider: ${name}. Registered: ${known}`);
+  }
+  return factory;
+}
+
+export function listProviderNames(): string[] {
+  return [...registry.keys()];
+}