feat: wire factory principles into content index and write-skill governance check

- AGENTS.md: add factory principles and factory-integration-decisions to
  on-demand content index so they are discoverable for skill design work
- write-skill: insert conflict-check step (step 3) — sub-agent reads
  constitution and factory principles after grill, before any writing;
  hard gate with explicit self-check item; renumber steps 3–8 → 4–9
- META.md: bump to v1.4

Closes the gap identified in PRD chunk-3-skills-library §sub-agent-usage:
conflict checking against constitution and factory principles was specified
but never wired into the write-skill process.

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

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

@@ -1,5 +1,5 @@
 ```yaml
-version: "1.3"
+version: "1.4"
 updated: 2026-05-26
 
 # when: describes when this skill is loaded — the full trigger context.