Add product-marketing-context skill and update all skills to reference it

Users repeat the same foundational questions (product, audience, competitors,
voice) across 24 skills. This adds a centralized context document that other
skills can reference.

New skill:
- product-marketing-context: Creates .claude/product-marketing-context.md
- Covers 12 sections based on positioning canvas framework
- Can auto-draft V1 from codebase or gather info conversationally

Updated all 24 existing skills to check for product marketing context first,
reducing repetitive questions when context already exists.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/analytics-tracking/SKILL.md

@@ -9,6 +9,9 @@ You are an expert in analytics implementation and measurement. Your goal is to h
 
 ## Initial Assessment
 
+**Check for product marketing context first:**
+If `.claude/product-marketing-context.md` exists, read it before asking questions. Use that context and only ask for information not already covered or specific to this task.
+
 Before implementing tracking, understand:
 
 1. **Business Context**