[FEATURE] Sort agents alphabetically in /agents dialog

Problem Statement

When opening the /agents dialog, the available agents are listed in no apparent order. With 10+ custom agents defined (which is common when using agent frameworks like gstack), finding a specific agent requires scanning the entire unordered list every time.

This becomes a real friction point during fast-paced development sessions where you're switching between agents frequently.

Proposed Solution

Sort the agent list alphabetically by agent name in the /agents dialog. This is the simplest change that makes the list scannable.

Bonus: a search/filter input at the top of the dialog (like the / command palette) would make it even faster for users with many agents.

Alternative Solutions

• Manually renaming agents with numeric prefixes to force ordering (hacky, breaks naming conventions)
• Memorizing the arbitrary order (doesn't scale)
• Typing the agent name directly instead of using the dialog (works but defeats the purpose of the dialog)

Use Case Example

1. I have 20+ specialized agents defined (code review, QA, design audit, deployment, etc.)
2. I open /agents to pick the right one for my current task
3. Currently: I scan an unordered list of 20+ items trying to find "QA" or "Backend Architect"
4. With this feature: I'd immediately know to look in the Q or B section of an alphabetical list
5. This saves 5-10 seconds per agent selection, which adds up across a session

Additional Context

Priority: Medium - Would be very helpful
Category: Interactive mode (TUI)

This is a small UX improvement that would benefit anyone using custom agents at scale. The implementation should be straightforward — just sort the array before rendering.