fix: write-skill progressive disclosure — sub-file structure and lessons

- Add sub-file constraint: content-type split rule (explains vs. directs),
  three spec-defined optional directories (scripts/, references/, assets/),
  one-level-deep rule, and wiring syntax requirement
- Update output format section to list optional sub-files as a third output
- Add self-check item for sub-file placement and wiring
- Update SKILL-TEMPLATE.md constraints and output format examples to match
- Bump META.md to v1.3
- Add two LESSONS.md entries: research agents presenting synthesis as spec
  fact; META-TEMPLATE fix deferred with explicit do-not-apply note

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

.agents/skills/write-skill/META.md

@@ -1,5 +1,5 @@
 ```yaml
-version: "1.2"
+version: "1.3"
 updated: 2026-05-26
 
 # when: describes when this skill is loaded — the full trigger context.