skills: split oversized SKILL.md files into references/ (f3s pattern)

Break up the three most monolithic flat skills into a slim SKILL.md plus a
references/ directory of focused sub-files, matching the f3s and
agent-task-management structure. Pure reorganization: all content and meaning
preserved, only moved behind a reference index with resolving links.

- blog-writing-style (619->216): extract the Wikipedia-based 'Signs of AI
  writing' deep catalog into references/signs-of-ai-writing.md.
- check-shopping-status (319->135): extract references/imap-scan-script.md
  (steps 1-3 Python) and references/amazon-cdp.md (CDP login helper).
- photo-processing (232->155): extract references/auto-enhance.md (standalone
  ImageMagick auto-enhance script).

Adds notes/skills-review-2026-06-19.md with the full inventory and the
follow-up task IDs (dq0, eq0, fq0) for the remaining oversized skills.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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