feat: add tmux session preview panel

[renato0307]

Summary

Add a toggleable preview panel that displays the content of the selected tmux session's active pane.

Behavior

• Toggle: Press v to show/hide the preview panel
• Layout: When visible, splits the content area 50/50 with the session list
• Content: Shows the active pane of the currently selected session
• Auto-refresh: Uses fast polling (300-500ms) to keep content up-to-date
• Selection change: When selecting a different session, preview updates to show that session's content

Settings

Setting	Type	Default	Description
preview_layout	string	vertical	Layout direction: vertical (list top, preview bottom) or horizontal (list left, preview right)
preview_max_lines	int	50	Maximum lines to capture (actual lines shown depends on available terminal space)

Technical Approach

Capturing tmux content

# Get active window.pane for session
tmux display-message -p -t <session> '#{window_index}.#{pane_index}'

# Capture last N lines from active pane
tmux capture-pane -p -t <session>:<window>.<pane> -S -<lines>

Refresh strategy

Fast polling at 300-500ms interval while preview is visible. Simple, reliable, and effectively indistinguishable from true streaming for this use case.

UI Mockup

Vertical layout (default)

┌─────────────────────────────────┐
│  Session List                   │
│  ─────────────────────────────  │
│  > session-1  [Working]         │
│    session-2  [Idle]            │
│    session-3  [Waiting]         │
├─────────────────────────────────┤
│  Preview (session-1)            │
│  ─────────────────────────────  │
│  $ claude                       │
│  > Working on feature...        │
│  ...                            │
└─────────────────────────────────┘

Horizontal layout

┌────────────────┬────────────────┐
│  Session List  │  Preview       │
│  ────────────  │  ────────────  │
│  > session-1   │  $ claude      │
│    session-2   │  > Working...  │
│    session-3   │  ...           │
└────────────────┴────────────────┘

Tasks

• Add preview_layout and preview_max_lines to settings
• Add tmux client method to capture pane content
• Create preview panel UI component
• Implement toggle logic with v key
• Add polling mechanism for auto-refresh
• Update help screen with new shortcut
• Calculate visible lines based on available terminal space