CLI - Add interactive mode to agentstack run command

[jenna-winkler]

Description

Currently, running an agent requires users to manually type the exact agent name or ID they want to run:

agentstack run example-agent

Problem

Users must know the exact agent name ahead of time, meaning they generally have to run agentstack list first, which can be error prone and unintuitive, especially when multiple agents are available.

Proposed solution

Introduce an interactive mode for the agentstack run command that helps users select an agent to run from a list:

• Lists all agents available to the user
• Prompts the user to select one
• Executes the run operation

Example usage

$ agentstack run

? Select an agent to run:
  ❯ example-agent
    another-agent
    test-agent

Notes

• Maintain existing functionality where users can specify the agent upfront