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/utils.js

@@ -0,0 +1,29 @@
+/**
+ * Shared provider utilities.
+ *
+ * @module providers/utils
+ */
+
+/**
+ * Prefixes that indicate internal/system content which should be hidden from the UI.
+ * @type {readonly string[]}
+ */
+export const INTERNAL_CONTENT_PREFIXES = Object.freeze([
+  '<command-name>',
+  '<command-message>',
+  '<command-args>',
+  '<local-command-stdout>',
+  '<system-reminder>',
+  'Caveat:',
+  'This session is being continued from a previous',
+  '[Request interrupted',
+]);
+
+/**
+ * Check if user text content is internal/system that should be skipped.
+ * @param {string} content
+ * @returns {boolean}
+ */
+export function isInternalContent(content) {
+  return INTERNAL_CONTENT_PREFIXES.some(prefix => content.startsWith(prefix));
+}