feat(cli): implement `iterate agents list` — detect installed AI agent CLIs

[forgou37]

The sh1pt iterate agents list command currently prints a hardcoded stub list without checking whether any of the agent CLIs (claude, codex, qwen) are actually installed on the machine.

Proposed change

Call <binary> --version for each known agent and report real installation status:

$ sh1pt iterate agents list
  ● claude  v1.2.3
  ○ codex   not installed — npm install -g @openai/codex
  ○ qwen    not installed — pip install qwen-agent

1/3 agent(s) installed. Run `sh1pt agents setup` to install missing agents.

Also supports --json output for scripting.