Content quality improvements to remaining 5 engineering skills

Completes the quality pass across all 10 skills:
- incident-postmortem: fix opening paragraph (blameless framing emphasis),
  add root cause circular check + action item specificity quality checks
- pr-description-writer: add title format quality check, fix
  risk-appropriate reviewer guidance quality check
- system-design-interview: rewrite architecture diagram instruction
  (system-specific not generic template), fix capacity estimates to show
  arithmetic, add trade-offs non-empty check
- api-docs-writer: add API Version + Rate Limits inputs, clarify output
  format options, add error codes completeness check, fix code examples check
- architecture-decision-record: add ADR Number + Team Context inputs,
  fix Implementation Notes + Review Date guidance, fix quality checks for
  context specificity and rejected option reasoning

Both skills/ and plugins/pm-engineering/skills/ copies updated.

https://claude.ai/code/session_01C3HwChrccJd145vJ6Z7ajF

plugins/pm-engineering/skills/pr-description-writer/SKILL.md

@@ -78,8 +78,9 @@ Flag anything that warrants extra attention:
 - [ ] Title is imperative mood and under 72 characters
 - [ ] Summary explains what AND why (not just what)
 - [ ] Changes list describes logical changes (not file-by-file changes)
+- [ ] Title starts with a valid type prefix (feat / fix / refactor / chore / deps / config / hotfix) and is under 72 characters
 - [ ] Testing steps are reproducible by someone unfamiliar with the code
-- [ ] Risk-appropriate reviewer guidance is included
+- [ ] For high-risk PRs, Reviewer Notes flags at least one specific area of concern or deliberate trade-off; for low-risk PRs, Reviewer Notes is either omitted or kept to one line
 
 ## Usage Examples
 - "Write a PR description for these changes" + [paste diff or description]