Add Understanding extension to community catalog (#1778)

* Add Understanding extension to community catalog

31 deterministic requirements quality metrics based on IEEE/ISO standards.
Catches ambiguity, missing testability, and structural issues before implementation.
Includes experimental energy-based ambiguity detection.

Repository: https://github.com/Testimonial/understanding
Commands: scan, validate, energy
Hook: after_tasks validation prompt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Sort README table and catalog entries alphabetically

Move Understanding extension entry between Spec Sync and V-Model
to maintain alphabetical ordering in both files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Ladislav Bihari <ladislav.bihari@statsperform.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>