refactor(agent): consolidate agent system into unified AgentManager with provider-based architecture

Replace separate ACP and agent-types modules with unified AgentManager using provider pattern. Migrate Claude and Codex implementations to provider-specific clients. Streamline session-agent integration, remove old ACP notification types, and introduce agent timeline events. Delete obsolete documentation and test files. Add AgentRegistry for persistence and agent-stream-based communication pattern.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/server/package.json

@@ -17,13 +17,10 @@
     "test:e2e:mobile": "playwright test --project='Mobile Chrome'"
   },
   "dependencies": {
-    "@agentclientprotocol/sdk": "^0.4.9",
     "@ai-sdk/openai": "^2.0.52",
-    "@boudra/claude-code-acp": "^0.8.5",
     "@deepgram/sdk": "^3.4.0",
     "@modelcontextprotocol/sdk": "^1.20.1",
     "@openrouter/ai-sdk-provider": "^1.2.0",
-    "@zed-industries/codex-acp": "^0.4.0",
     "ai": "^5.0.76",
     "dotenv": "^17.2.3",
     "express": "^4.18.2",