UX: Status bar should show user's selected model, not subagent model

Problem

When using the /model command to select a model (e.g., Opus), the status bar changes to show a different model (e.g., Haiku) when subagents are spawned via the Task tool. This is confusing, especially for users who frequently switch models.

Current Behavior

• User selects Opus via /model
• Status bar shows "Opus"
• Task tool spawns a subagent with model: haiku
• Status bar changes to show "Haiku"
• User is confused about which model is actually their session model

Expected Behavior

• Main status bar: Always shows the user's selected session model (what they set via /model)
• Subagent status: When a subagent is running with a different model, show it separately, e.g., [Task: haiku] or Subagent (haiku)

Why This Matters

Users who switch models frequently using /model rely on the status bar to confirm their selection. When it changes unexpectedly due to subagent activity, it creates confusion about:

1. Whether their /model command worked
2. What model is actually processing their main conversation
3. Whether they need to re-run /model

Suggested Solution

Keep the primary model indicator stable and add a secondary indicator for subagent model usage when applicable.