Add configuration validation & better error messages

[Emperor42]

Shepherd.init only checks for three required fields. Enhance validation to:

Ensure visibleTagCount is a positive integer.
Verify tagSpacingPx is a non‑negative number.
Confirm each tag object has a non‑empty text string and a non‑empty keywords array.
Throw descriptive Errors (e.g., “Shepherd.init – tag[3] missing keywords”) so developers can debug quickly.