Improve secret-like scanner false-positive handling

[erkininfinity]

Goal

Improve the scanner so it catches real secret-like values while avoiding common documentation placeholders.

Suggested scope

• Add fixtures for placeholder values, real-looking keys, and docs examples.
• Expand tests around scanContent.
• Keep messages phrased as "possible secret-like value" to avoid overclaiming.

Why this helps

The scanner is security-sensitive and should be useful without being noisy.