feat: Disable default agents

[taqtiqa-mark]

Given I am an IDE user.
When I want to use a primary agent, `agent/ide.md`, specialized to my use case.
Then I want to eliminate TUI static by disabling default agents by setting `default_agents: false`.
And then when OC loads the active agent display shows `IDE AGENT`.
And then pressing `tab` does not rotate to any other agent.

Given I am an IDE user.
When I want to use the primary agents, `agent/ide.md`, `agent/ta.md`, specialized to my different use cases.
Then I want to eliminate TUI static by disabling default agents by setting `default_agents: false`.
And then I want to set `default_agent: ta`.
And then OC loads with the active agent display showing `TA AGENT`.
And then pressing `tab` rotates to the IDE agent.
And then the active agent display shows `IDE AGENT`.

[github-actions]

This issue might be a duplicate or related to existing issues. Please check:

• Switch model for all agents #3028: Switch model for all agents - discusses agent switching behavior and UX improvements
• agents switch does not respect the custom model with id #3015: agents switch does not respect the custom model with id - deals with agent switching and custom configurations
• PLAN agent specific permission #2989: PLAN agent specific permission - discusses agent-specific configurations and permissions

While these don't directly address disabling default agents, they involve similar concepts around agent configuration and customization. Feel free to ignore if none of these address your specific case.

[taqtiqa-mark]

@rekram1-node not sure if this is covered by test suite, but a workaround is config.json:

...
  "agent": {
    "plan": {
      "disable": true
    },
    "build": {
      "disable": true
    }
  },
...  

[rekram1-node]

@taqtiqa-mark yeah I think we added that at some point recently and I didn't close this one out

[taqtiqa-mark]

Note to self: Add general agent to the list to disable...