Add real-time Amazon Q context window usage to tmux status bar

## Goal

Display live Amazon Q CLI context window usage in the tmux status bar for the currently focused pane.

## Desired Result

When working in a pane with an active Amazon Q CLI session, the tmux status bar should show:
```
Q: 127.6k/200k (64%)
```

With color coding:
- Green: < 60% usage
- Yellow: 60-80% usage
- Red: > 80% usage

## Benefits

- **Fast feedback loop**: Watch token usage increment in real-time as you interact with Amazon Q
- **Context awareness**: Know when approaching token limits without manual `/usage` commands
- **Joyful development**: Ambient awareness that enhances rather than interrupts workflow
- **Pane-specific**: Only shows when focused pane has active Amazon Q session

## Success Criteria

- [ ] Status updates frequently during active Q sessions
- [ ] Non-intrusive to ongoing conversations
- [ ] Visual feedback shows usage trending up/down
- [ ] Follows "Keep joy in the loop" DevEx principle

Start by locating the tmux status-bar configuration in this dotfiles repository and determine how the focused pane can be associated with an active Amazon Q CLI session. Investigate whether live context-window usage is exposed, then define refresh timing, percentage color thresholds, and the behavior when no Q session is present.

Written by the indexing model from the issue text.