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.