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>

src/providers/index.ts

@@ -0,0 +1,6 @@
+// Host-side provider container-config barrel.
+// Providers that need host-side container setup (extra mounts, env passthrough,
+// per-session directories) self-register on import. Providers with no host
+// needs (claude, mock) don't appear here.
+//
+// Skills add a new provider by appending one import line below.

src/providers/provider-container-registry.ts

@@ -0,0 +1,63 @@
+/**
+ * Host-side provider container-config registry.
+ *
+ * Providers that need per-spawn host-side setup (extra volume mounts, env var
+ * passthrough, per-session directories) register a function here. The
+ * container-runner resolves the session's effective provider name, looks up
+ * the registered config fn, and merges the returned mounts/env into the spawn
+ * args.
+ *
+ * Providers without host-side needs (e.g. `claude`, `mock`) don't appear in
+ * this registry at all — the lookup returns `undefined` and the spawn path
+ * proceeds with only the default mounts and env.
+ *
+ * Skills add a new provider's host config by creating `src/providers/<name>.ts`
+ * with a top-level `registerProviderContainerConfig(...)` call, then appending
+ * `import './<name>.js';` to `src/providers/index.ts` (the barrel).
+ */
+
+export interface VolumeMount {
+  hostPath: string;
+  containerPath: string;
+  readonly: boolean;
+}
+
+export interface ProviderContainerContext {
+  /** Per-session host directory: `<DATA_DIR>/v2-sessions/<session_id>`. */
+  sessionDir: string;
+  /** Agent group ID, for any per-group logic. */
+  agentGroupId: string;
+  /** `process.env` at spawn time — pull passthrough values from here. */
+  hostEnv: NodeJS.ProcessEnv;
+}
+
+export interface ProviderContainerContribution {
+  /** Extra volume mounts (merged with the default session/group/agent-runner mounts). */
+  mounts?: VolumeMount[];
+  /** Extra env vars to pass to the container (`-e KEY=VALUE`). */
+  env?: Record<string, string>;
+}
+
+export type ProviderContainerConfigFn = (
+  ctx: ProviderContainerContext,
+) => ProviderContainerContribution;
+
+const registry = new Map<string, ProviderContainerConfigFn>();
+
+export function registerProviderContainerConfig(
+  name: string,
+  fn: ProviderContainerConfigFn,
+): void {
+  if (registry.has(name)) {
+    throw new Error(`Provider container config already registered: ${name}`);
+  }
+  registry.set(name, fn);
+}
+
+export function getProviderContainerConfig(name: string): ProviderContainerConfigFn | undefined {
+  return registry.get(name);
+}
+
+export function listProviderContainerConfigNames(): string[] {
+  return [...registry.keys()];
+}