docs: apply awesome-readme best practices

Major README improvements based on analysis of top-tier README examples
(size-limit, ky) and awesome-readme repository.

## Changes Applied

### 1. Problem-Solution Opening ✨
**Before**: Generic tagline
**After**: Problem → Solution framing
- Identifies pain point (commands in isolation)
- Shows consequence (freeze in real codebase)
- Presents solution (workflow training)

### 2. Quick Links Section 🔗
Added prominent navigation after header:
- Download, Quick Start, Features, Docs, Issues, Contributing
- Single click to key sections
- Reduces time-to-action

### 3. Consolidated Features Section ✨
**Before**: Features scattered across multiple sections
**After**: Single scannable list with 8 key features
- Smart Learning (FSRS)
- Daily Quests
- Progress Tracking
- Offline & Private
- Real Helix Accuracy
- 31 Commands
- Gamification
- 20 Scenarios

### 4. Improved Badges 📊
- Added CI Status badge (shows build health)
- Removed Edition badge (too technical)
- Kept License and Rust version (essential)

### 5. Code Example with Real Output 💻
Added visual example of training session:
- Shows actual TUI interface
- Demonstrates quest progress
- Shows XP and level system
- Makes value prop tangible

### 6. Extracted CONTRIBUTING.md 📝
**Before**: 92 lines in README
**After**: Compact summary (11 lines) + full CONTRIBUTING.md (104 lines)
- Cleaner main README
- Better for contributors (dedicated file)
- Follows GitHub conventions

### 7. Fixed Details/Summary Structure 🔧
**Before**: <details><summary><h2>Title</h2></summary>
**After**: ## Title\n<details><summary>Click to expand</summary>
- Preserves visual hierarchy
- Headings always visible
- Better GitHub rendering

## Impact

**Readability**:
- ✅ Problem-solution hook (30 sec to 'aha' moment)
- ✅ Quick Links (instant navigation)
- ✅ Visual example (shows, not tells)
- ✅ Scannable features (5 sec to understand value)

**Before**: 565 lines, philosophy-first, no examples
**After**: 599 lines, value-first, visual examples

**Structure**:
- Hook → Features → Quick Start → Details
- Show → Tell → Deep Dive
- Problem → Solution → How To Use

## Inspiration

Based on analysis of:
- awesome-readme repository
- size-limit (problem-solution framing, social proof)
- ky (minimalist badges, code examples)

## Files

- README.md: Optimized main documentation
- CONTRIBUTING.md: Extracted contribution guidelines

Analysis saved in .local/README_IMPROVEMENTS.md

Author: bug-ops