Add chat history method to continue previous agents chat created in different client

[remorses]

Add session history endpoints to ACP, enabling clients to access conversations from any ACP agent (Claude Code, Gemini CLI, etc).

Problem

Users work with AI agents across multiple tools, creating isolated conversation histories. Current ACP only handles active sessions, not historical data.

Proposed API

1. List Sessions

// Request
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "session/list",
  "params": {
    "limit": 20,
    "offset": 0,
    "search": "debugging"
  }
}

// Response
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "sessions": [{
      "sessionId": "sess_abc123",
      "title": "Fix Redis timeout",
      "createdAt": "2024-01-15T10:30:00Z",
      "lastModified": "2024-01-15T11:45:00Z",
      "source": "claude-code-cli",
      "messageCount": 12
    }],
    "total": 145
  }
}

2. Get Session Content

// Request
{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "session/get",
  "params": {
    "sessionId": "sess_abc123"
  }
}

// Response - Uses existing ACP content format
{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "sessionId": "sess_abc123",
    "title": "Fix Redis timeout",
    "createdAt": "2024-01-15T10:30:00Z",
    "turns": [
      {
        "role": "user",
        "timestamp": "2024-01-15T10:30:00Z",
        "content": [{
          "type": "text",
          "text": "Redis keeps timing out"
        }]
      },
      {
        "role": "assistant", 
        "timestamp": "2024-01-15T10:30:15Z",
        "content": [{
          "type": "text",
          "text": "I'll check your Redis config"
        }],
        "toolCalls": [{
          "toolCallId": "call_001",
          "title": "Read redis.conf",
          "status": "completed"
        }]
      }
    ]
  }
}

3. Capability Discovery

{
  "jsonrpc": "2.0",
  "id": 0,
  "result": {
    "protocolVersion": 1,
    "agentCapabilities": {
      "sessionHistory": {
        "supported": true,
        "maxResults": 1000
      }
    }
  }
}

Benefits

• Unified history across all AI tools
• Context reuse for similar problems
• Backwards compatible - optional feature
• Simple implementation - reuses existing content types

Implementation

Agents store sessions locally (SQLite, JSON files) and expose via new endpoints. No changes to existing ACP flows.

[agu-z]

It isn't implemented in clients yet, but we already have session/load (docs), which uses the existing notifications to stream the conversation as if they were the result of session/prompt. This makes it much easier to implement, as the Client can collect the content in the same way they already do, and we don't need to send all content at once in a single message.

Adding something like session/list would make a lot of sense because it means the Client doesn't have to keep track of past sessions and can discover ones from other clients. So I'm in favor, but there are a few other features we want to stabilize first.