herdr tmux/cmux: Recognize Command Code (cmd) as an agent with notification support

Feature Description

Add first-class support for Command Code (cmd) as a recognized agent type within herdr when running inside tmux or cmux terminal sessions.

Currently, herdr detects and manages the following agents correctly:

• antigravity
• codex
• claude etc

However, cmd (Command Code) agents are not recognized by herdr in the same terminal context. They run silently — not listed in herdr's agent registry, and no notifications fire in herdr or cmux when they are triggered.

The fix should include:

1. Adding cmd / Command Code to herdr's agent detection/recognition logic
2. Enabling notification hooks for cmd agents in both herdr and cmux (on par with metoo notification behavior for existing agents)
3. Ensuring full parity with claude, codex, and antigravity agent handling inside tmux/cmux sessions

Use Case

Developers using herdr to orchestrate multiple AI agents inside tmux or cmux sessions need Command Code agents to be visible and manageable alongside other agents (claude, codex, antigravity).

Without this support:

• cmd agents run as untracked processes — no status visibility in herdr
• No notifications fire when a Command Code agent completes a task or needs attention
• Breaks multi-agent workflows where herdr is the central orchestration layer

With this support, Command Code becomes a first-class citizen in herdr's agent ecosystem, enabling seamless use alongside other recognized agents in terminal-based multi-agent setups.

Additional Context

https://herdr.dev https://cmux.com

How important is this to you?

Blocking adoption or production use

首先定位 herdr 在 tmux 或 cmux 会话中针对 claude、codex、antigravity 和 metoo 行为的 agent 检测与 registry 逻辑。跟踪通知钩子，并验证 cmd 能够被识别、列出并以等效行为发送通知；在 herdr 和 cmux 工作流中都确认结果。