Add AUTO mode with automatic timestamp detection

This commit adds intelligent auto-detection of data age, automatically
choosing the appropriate ingestion method without user intervention.

## New Features

1. **AUTO Mode** (-mode=auto)
   - Automatically detects timestamp age from input data
   - Routes realtime data (< 5min) → Pushgateway
   - Routes historic data (> 5min) → Remote Write API
   - No manual timestamp calculation needed!

2. **Input Format Support**
   - CSV format: metric_name,labels,value,timestamp_ms
   - JSON format: [{metric, labels, value, timestamp_ms}]
   - Read from file (-file=path) or stdin
   - Comments supported in CSV (#)

3. **Smart Routing Logic**
   - 5-minute threshold determines ingestion method
   - Handles mixed data (current + historic) in single import
   - Clear logging shows which method is used for each sample

4. **Test Data Generation**
   - generate-test-data.sh creates samples for all time ranges
   - Demonstrates: current, 1h, 1d, 1w, 1m old data
   - Actual timestamps generated dynamically

## Files Added

- auto-ingest.go: Core auto-detection logic
- AUTO-MODE.md: Complete documentation
- generate-test-data.sh: Test data generator
- test-data.csv: Example data template
- test-all-ages.csv: Generated test data (all ages)

## Example Usage

```bash
# Generate test data
./generate-test-data.sh

# Auto-import (detects ages automatically)
./prometheus-pusher-auto \
  -mode=auto \
  -file=test-all-ages.csv \
  -pushgateway=http://localhost:9091 \
  -prometheus=http://localhost:9090/api/v1/write
```

## Output Example

```
📊 Auto-ingest summary:
  Total samples: 15
  Realtime samples (< 5min old): 3
  Historic samples (> 5min old): 12

🔄 Ingesting 3 REALTIME samples via Pushgateway...
⏰ Ingesting 12 HISTORIC samples via Remote Write...
  [1/12] app_requests_total (age: 1.0 hours)
  [2/12] app_temperature_celsius (age: 1.0 days)
  ...
🎉 Auto-ingest complete!
```

## Supported Time Ranges

✅ Current data (< 5min)
✅ 1 hour old data
✅ 1 day old data
✅ 1 week old data
✅ 1 month old data

All ages are automatically detected and routed correctly!

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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