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';