ref(skill): Extract detailed docs to supporting files

Refactor dex skill following best practices guidance to keep SKILL.md
under 500 lines. Moved detailed reference material to supporting files
that Claude loads when needed:

- cli-reference.md: Full CLI docs, flags, storage format
- examples.md: Good/bad context and result examples
- verification.md: Verification checklist and process
- hierarchies.md: Epic/task/subtask organization

SKILL.md reduced from ~655 to 146 lines, focusing on core concepts
and workflow guidance. Claude loads supporting files on demand.

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

Author: dcramer