feat: 100 skills milestone — 7 new skills + quality improvements across all 93

New skills added:
- teaching-lesson-plan: structured lesson plans for any subject/audience/setting
- seo-content-brief: complete SEO briefs with intent, competitor gaps, and outline
- media-pitch: story-first journalist pitches with angle development framework
- change-management-plan: stakeholder analysis, comms strategy, adoption metrics
- workshop-facilitation-guide: activity instructions, decision protocols, facilitator moves
- sales-forecasting-model: pipeline model, scenario analysis, assumption log
- tax-planning-checklist: year-end tax planning across income, pension, CGT, reliefs

Quality improvements across all 93 existing skills:
- Standardised description format: "Verb the thing. Use when X. Produces Y."
- Added Required Inputs section to all skills missing it (prompts for missing info)
- Added Quality Checks section to all skills missing it (specific, not generic)
- Fixed broken multiline YAML descriptions
- Removed non-standard frontmatter keys (tool_integration, metadata blocks)

README updated to v6.0.0 with 100-skill count, new skill tables, and article series

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

skills/onboarding-plan/SKILL.md

@@ -83,6 +83,14 @@ Goals:
 Manager: Meeting expectations? What to double down on? What to develop?
 New hire: Have the clarity, tools, support needed? What surprised you? What would you change about onboarding?
 
+## Quality Checks
+
+- [ ] Before Day 1 manager checklist is complete (IT, access, buddy, calendar)
+- [ ] Each phase (orient/learn/contribute/lead) has a clear milestone
+- [ ] 90-day review questions are included for both manager and new hire
+- [ ] Plan is tailored to the specific role and level (not generic)
+- [ ] Key stakeholder 1:1s are listed by name or role
+
 ## Example Trigger Phrases
 - "Create a 30/60/90 day plan for a new [role]"
 - "Write an onboarding plan for [name] starting as [role]"