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/proposal-writer/SKILL.md

@@ -86,6 +86,14 @@ Writes commercial proposals that win business — structured around the prospect
 2. We will send contract and confirm kickoff
 3. [Any immediate action]
 
+## Quality Checks
+
+- [ ] "Understanding Your Situation" reflects what was learned in discovery (not generic)
+- [ ] Outcomes are listed (not just deliverables or features)
+- [ ] "Not included" section is explicit to prevent scope disputes later
+- [ ] Next steps include a specific date and named action
+- [ ] "Valid until" date is included to create urgency
+
 ## Example Trigger Phrases
 - "Write a proposal for [prospect] to [solve their problem]"
 - "Draft a statement of work for [project]"