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/routes/codex.js

@@ -4,7 +4,7 @@ import { promises as fs } from 'fs';
 import path from 'path';
 import os from 'os';
 import TOML from '@iarna/toml';
-import { getCodexSessions, getCodexSessionMessages, deleteCodexSession } from '../projects.js';
+import { getCodexSessions, deleteCodexSession } from '../projects.js';
 import { applyCustomSessionNames, sessionNamesDb } from '../database/db.js';
 
 const router = express.Router();
@@ -68,24 +68,6 @@ router.get('/sessions', async (req, res) => {
   }
 });
 
-router.get('/sessions/:sessionId/messages', async (req, res) => {
-  try {
-    const { sessionId } = req.params;
-    const { limit, offset } = req.query;
-
-    const result = await getCodexSessionMessages(
-      sessionId,
-      limit ? parseInt(limit, 10) : null,
-      offset ? parseInt(offset, 10) : 0
-    );
-
-    res.json({ success: true, ...result });
-  } catch (error) {
-    console.error('Error fetching Codex session messages:', error);
-    res.status(500).json({ success: false, error: error.message });
-  }
-});
-
 router.delete('/sessions/:sessionId', async (req, res) => {
   try {
     const { sessionId } = req.params;