feat: Path based loading for agent

[mekza]

New version of #2859 after fixing the tests:

This PR transforms how users organize and reference their Q CLI agents by introducing hierarchical directory support. Previously, all agents lived in a flat directory structure, forcing users to rely solely on unique names to avoid collisions. Now, agents can be organized in subdirectories and referenced using intuitive path notation.

cli-agents/
  team/
    backend-expert.json
    frontend-specialist.json
  personal/
    code-reviewer.json
  shared/
    documentation-helper.json

It enables users to reference agents as q chat --agent team/backend-expert in addition to q chat --agent backend-expert. Users can now organize agents hierarchically in the Q CLI directory structure, avoiding naming collisions and improving discoverability through logical grouping (e.g., team-specific, project-specific, or role-specific agent directories).

[adamalrefai]

Does this PR also add support for loading an agent with a path to the config file? So for example q chat --agent /path/to/agent.json

[mekza]

@adamalrefai not full path only path relative to $HOME/.aws/amazonq/cli-agents

[adamalrefai]

@mekza Alright, got it :) This is really good and really needed, but was hoping that this will also add support for global path picking.. I think that should be brought up, the /agent swap should also support that in my opinion, like /agent swap --path /path/to/agent.json