refactor(agent-modal): revamped UI/UX for the agent modal

[spoons-and-mirrors]

Summary

• Refactored the TUI agent modal for more consistent UI with models modal
• Improved user experience with Recent (max 5), Agents, and Subagents categories
• In list coloring of agent names with their respective color
• Exposing Subagents in the list so users can use them directly (resolves half of #1832)
• Future proofing UX for users with many agents and subagents

Note

Plan and Build agents description could be added for consistency, I'll leave this for a later date

[github-actions]

Found several violations of AGENTS.md guidelines:

1. Line 319 in app.go: Variable name agentName violates "PREFER single word variable names where possible". Consider renaming to agent or name.

2. Line 274 in app.go: Variable name agentName violates the single word variable naming guideline.

3. Line 39 in app.go: Variable name agentUsage could be simplified to usage.

4. Line 90 in state.go: Variable name agentName should be agent or name per guidelines.

5. Line 331-337 in agents.go: The nested if-else structure could be simplified by removing unnecessary else statements as per "AVOID else statements".

These changes would improve code consistency with the project guidelines.