feat(provider-adapter): implement multi-provider abstraction layer (Phase 1)

Add provider types, registry, capabilities, and message adapter registry.
Rename sdk-messages→claude-messages, agent-runner→claude-runner,
ClaudeSession→AgentSession. Update Rust AgentQueryOptions with provider
and provider_config fields. Capability-driven AgentPane rendering.

v2/src/lib/adapters/message-adapters.ts

@@ -0,0 +1,29 @@
+// Message Adapter Registry — routes raw provider messages to the correct parser
+// Each provider registers its own adapter; the dispatcher calls adaptMessage()
+
+import type { AgentMessage } from './claude-messages';
+import type { ProviderId } from '../providers/types';
+import { adaptSDKMessage } from './claude-messages';
+
+/** Function signature for a provider message adapter */
+export type MessageAdapter = (raw: Record<string, unknown>) => AgentMessage[];
+
+const adapters = new Map<ProviderId, MessageAdapter>();
+
+/** Register a message adapter for a provider */
+export function registerMessageAdapter(providerId: ProviderId, adapter: MessageAdapter): void {
+  adapters.set(providerId, adapter);
+}
+
+/** Adapt a raw message using the appropriate provider adapter */
+export function adaptMessage(providerId: ProviderId, raw: Record<string, unknown>): AgentMessage[] {
+  const adapter = adapters.get(providerId);
+  if (!adapter) {
+    console.warn(`No message adapter for provider: ${providerId}, falling back to claude`);
+    return adaptSDKMessage(raw);
+  }
+  return adapter(raw);
+}
+
+// Register Claude adapter by default
+registerMessageAdapter('claude', adaptSDKMessage);