[Feature Request] Group chat support + per-chat session isolation (一个群聊一个会话，多会话并行互不干扰)

[olaycc37-cyber]

Feature Request: Group Chat Support + Per-Chat Session Isolation

Problem / Current Limitation

LarkRemote currently works great for single-user private chat with the bot. However, it has two architectural limitations that prevent team/multi-session workflows:

1. Observation & Takeover states are global singletons

The following state files are shared across ALL chats (not isolated per chat_id):

• observation.json — only ONE session can be observed at a time, globally
• takeover.json — only ONE session can be taken over at a time, globally
• handoff.json — single global handoff state

This means:

• If I'm observing session A in private chat, and a colleague (or myself in a group) tries to observe session B, it overwrites my observation of session A.
• Cannot run multiple parallel observations/takeovers for different Codex sessions simultaneously.

2. No group chat support

• The bot can technically be added to a Feishu group (it receives a chat_id), but the permission model is based on senderId (user-level), not designed for multi-user group scenarios.
• There's no mechanism to bind one group/thread to one specific Codex session.

Desired Behavior

Goal: One group chat (or thread) = One Codex session, fully isolated.

Similar to how lark-coding-agent-bridge works (but that project only supports Codex CLI, not Codex Desktop):

1. Bot can be added to a Feishu group

   • The group is bound to a specific project (working directory)
   • Optionally: within a group, each thread (话题) maps to an independent Codex session

2. Per-chat state isolation

   • Each chat_id (private chat, group, or thread) has its OWN independent:
     • observation.json → which session this chat is observing
     • takeover.json → which session this chat has taken over
     • message routing state
   • User A in private chat observing session X does NOT conflict with Group B observing session Y

3. Auto-create dedicated chat for a task (optional stretch goal)

   • Command like /new-chat <task-name> creates a new Feishu group/chat dedicated to one Codex session
   • All progress notifications for that session go to that dedicated chat
   • Users can jump into any chat to interact with that specific session

Use Cases

1. Parallel task monitoring: I run 3 Codex Desktop sessions on 3 different projects. I want to create 3 Feishu groups, each receiving notifications from ONE session, so I can check each task's status without mixing them up.

2. Team collaboration: Team lead adds the bot to a project group. Multiple team members can see the Codex session progress in that group. The session binding is stable and doesn't get hijacked by other chats.

3. Per-task dedicated channel: For a long-running refactor task, I create a dedicated Feishu group "refactor-auth-module". All notifications about that task go there. When the task completes/pauses/errors, the notification appears in that group with full context.

Technical Analysis / Suggested Implementation

I've reviewed the source code (v0.2.6) and believe the change scope is manageable:

Key files to modify:

1. src/observer.mjs — CodexSessionObserver currently loads/saves a single global observation.json. Change to per-chat storage: