feat: add Agent Reasoning Trace Explorer interactive demo

Interactive visualization of AI agent chain-of-thought reasoning:
- 5 scenario presets (simple query, research comparison, code debugging,
  ethical dilemma, route optimization)
- Animated step-by-step playback with speed controls (1x/2x/4x)
- Reasoning tree with expandable branches showing chosen/rejected/backtracked paths
- Confidence heatmap coloring (green/yellow/red)
- Click any node for detailed inspection (thought, confidence, tokens, duration)
- Live summary stats (steps, backtracks, max depth, avg confidence, tokens)
- Token usage bar chart by step type
- Confidence filter (all/high/low)
- Responsive layout, matches site dark theme

Usage: Open reasoning-trace.html or navigate from the main site.

Author: sauravbhattacharya001