docs: fix report walkthrough numbers to match actual 06_local_data output

The walkthrough example in read-reports.md had hardcoded numbers
from an old losing strategy, but the referenced example
(06_local_data.py) is actually a modestly profitable MA crossover
strategy. Updated all metrics to match the actual output:
- Total Return: +16.11% (was -33.28%)
- Sharpe: +0.22 (was -2.01)
- Max Drawdown: -14.45% (was -38.32%)
- etc.

Also updated the narrative from "losing strategy" to "modestly
profitable strategy" to match reality.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>