feat: unified message architecture with provider adapters and session store

- Add provider adapter layer (server/providers/) with registry pattern
    - Claude, Cursor, Codex, Gemini adapters normalize native formats to NormalizedMessage
    - Shared types.js defines ProviderAdapter interface and message kinds
    - Registry enables polymorphic provider lookup

  - Add unified REST endpoint: GET /api/sessions/:id/messages?provider=...
    - Replaces four provider-specific message endpoints with one
    - Delegates to provider adapters via registry

  - Add frontend session-keyed store (useSessionStore)
    - Per-session Map with serverMessages/realtimeMessages/merged
    - Dedup by ID, stale threshold for re-fetch, background session accumulation
    - No localStorage for messages — backend JSONL is source of truth

  - Add normalizedToChatMessages converter (useChatMessages)
    - Converts NormalizedMessage[] to existing ChatMessage[] UI format

  - Wire unified store into ChatInterface, useChatSessionState, useChatRealtimeHandlers
    - Session switch uses store cache for instant render
    - Background WebSocket messages routed to correct session slot

server/providers/registry.js

@@ -0,0 +1,44 @@
+/**
+ * Provider Registry
+ *
+ * Centralizes provider adapter lookup. All code that needs a provider adapter
+ * should go through this registry instead of importing individual adapters directly.
+ *
+ * @module providers/registry
+ */
+
+import { claudeAdapter } from './claude/adapter.js';
+import { cursorAdapter } from './cursor/adapter.js';
+import { codexAdapter } from './codex/adapter.js';
+import { geminiAdapter } from './gemini/adapter.js';
+
+/**
+ * @typedef {import('./types.js').ProviderAdapter} ProviderAdapter
+ * @typedef {import('./types.js').SessionProvider} SessionProvider
+ */
+
+/** @type {Map<string, ProviderAdapter>} */
+const providers = new Map();
+
+// Register built-in providers
+providers.set('claude', claudeAdapter);
+providers.set('cursor', cursorAdapter);
+providers.set('codex', codexAdapter);
+providers.set('gemini', geminiAdapter);
+
+/**
+ * Get a provider adapter by name.
+ * @param {string} name - Provider name (e.g., 'claude', 'cursor', 'codex', 'gemini')
+ * @returns {ProviderAdapter | undefined}
+ */
+export function getProvider(name) {
+  return providers.get(name);
+}
+
+/**
+ * Get all registered provider names.
+ * @returns {string[]}
+ */
+export function getAllProviders() {
+  return Array.from(providers.keys());
+}