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/index.ts

@@ -8,7 +8,8 @@
  *   - SESSION_INBOUND_DB_PATH:  path to host-owned inbound DB (default: /workspace/inbound.db)
  *   - SESSION_OUTBOUND_DB_PATH: path to container-owned outbound DB (default: /workspace/outbound.db)
  *   - SESSION_HEARTBEAT_PATH:   heartbeat file path (default: /workspace/.heartbeat)
- *   - AGENT_PROVIDER: 'claude' | 'mock' (default: claude)
+ *   - AGENT_PROVIDER: any registered provider name (default: claude). The
+ *     set of registered providers is whatever `providers/index.ts` imports.
  *   - NANOCLAW_ASSISTANT_NAME: assistant name for transcript archiving
  *   - NANOCLAW_ADMIN_USER_IDS: comma-separated user IDs allowed to run admin commands
  *
@@ -27,6 +28,9 @@ import path from 'path';
 import { fileURLToPath } from 'url';
 
 import { buildSystemPromptAddendum } from './destinations.js';
+// Providers barrel — each enabled provider self-registers on import.
+// Provider skills append imports to providers/index.ts.
+import './providers/index.js';
 import { createProvider, type ProviderName } from './providers/factory.js';
 import { runPollLoop } from './poll-loop.js';
 

container/agent-runner/src/providers/claude.ts

@@ -3,6 +3,7 @@ import path from 'path';
 
 import { query as sdkQuery, type HookCallback, type PreCompactHookInput } from '@anthropic-ai/claude-agent-sdk';
 
+import { registerProvider } from './provider-registry.js';
 import type { AgentProvider, AgentQuery, McpServerConfig, ProviderEvent, ProviderOptions, QueryInput } from './types.js';
 
 function log(msg: string): void {
@@ -271,3 +272,5 @@ export class ClaudeProvider implements AgentProvider {
     };
   }
 }
+
+registerProvider('claude', (opts) => new ClaudeProvider(opts));

container/agent-runner/src/providers/factory.ts

@@ -1,16 +1,13 @@
 import type { AgentProvider, ProviderOptions } from './types.js';
-import { ClaudeProvider } from './claude.js';
-import { MockProvider } from './mock.js';
+import { getProviderFactory } from './provider-registry.js';
 
-export type ProviderName = 'claude' | 'mock';
+/**
+ * Any registered provider name. Kept as a named alias for readability; the
+ * set of valid names is open and determined at runtime by whichever provider
+ * modules the `providers/index.ts` barrel imports.
+ */
+export type ProviderName = string;
 
 export function createProvider(name: ProviderName, options: ProviderOptions = {}): AgentProvider {
-  switch (name) {
-    case 'claude':
-      return new ClaudeProvider(options);
-    case 'mock':
-      return new MockProvider(options);
-    default:
-      throw new Error(`Unknown provider: ${name}`);
-  }
+  return getProviderFactory(name)(options);
 }

container/agent-runner/src/providers/index.ts

@@ -0,0 +1,6 @@
+// Provider self-registration barrel.
+// Each import triggers the provider module's registerProvider() call at top
+// level. Skills add a new provider by appending one import line below.
+
+import './claude.js';
+import './mock.js';

container/agent-runner/src/providers/mock.ts

@@ -1,3 +1,4 @@
+import { registerProvider } from './provider-registry.js';
 import type { AgentProvider, AgentQuery, ProviderEvent, ProviderOptions, QueryInput } from './types.js';
 
 /**
@@ -72,3 +73,5 @@ export class MockProvider implements AgentProvider {
     };
   }
 }
+
+registerProvider('mock', (opts) => new MockProvider(opts));

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()];
+}