[B2a] Brain TUI stats panel (Ctrl+P popup)

[four-bytes-robby]

Purpose

Ctrl+P popup showing search analytics and project stats in a compact modal.

Design

Add BrainStatsPanel component in src/tui.tsx:

• Triggered by Ctrl+P keyboard shortcut (use opencode TUI keyboard hook or useEffect with keydown)
• Modal/popup layout showing:

  🧠 Brain Stats — {project}
  ─────────────────────────
  📊 Search Analytics (this session)
    Searches:    42
    Avg results: 8.3
    Avg score:   0.72
    Top query:   "symfony entity pattern"
    
  📁 Project Index
    Files:   142
    Chunks:  847
    Memories: 23
    KB entries: 5
    
  🎯 Feedback
    Reported: 12/42 (29%)
    Helpful:  10/12 (83%)
    Used:      8/12 (67%)
  ─────────────────────────
  [ESC to close]
  

Data sources:

• Search analytics from search_analytics table (B2b-core)
• Project stats from getProjectStats() (B2c)
• Feedback stats from search_analytics WHERE helpful IS NOT NULL

Acceptance

• Ctrl+P opens stats popup
• ESC closes it
• All counters reflect live data
• Works in both sidebar and home variants
• Graceful fallback when no analytics data yet