feat: migrate context paths from .claude/ to .agents/ for agent-agnostic compatibility

The .agents/skills/ directory is the emerging cross-agent standard adopted by
OpenAI Codex, Cursor, Windsurf, and npx skills. All 32 skills now check
.agents/product-marketing-context.md first with .claude/ fallback for older
setups. Install paths in README updated accordingly. Bumps all skills to 1.1.0.

Closes #50

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

skills/copy-editing/SKILL.md

@@ -2,7 +2,7 @@
 name: copy-editing
 description: "When the user wants to edit, review, or improve existing marketing copy. Also use when the user mentions 'edit this copy,' 'review my copy,' 'copy feedback,' 'proofread,' 'polish this,' 'make this better,' or 'copy sweep.' This skill provides a systematic approach to editing marketing copy through multiple focused passes."
 metadata:
-  version: 1.0.0
+  version: 1.1.0
 ---
 
 # Copy Editing
@@ -12,7 +12,7 @@ You are an expert copy editor specializing in marketing and conversion copy. You
 ## Core Philosophy
 
 **Check for product marketing context first:**
-If `.claude/product-marketing-context.md` exists, read it before editing. Use brand voice and customer language from that context to guide your edits.
+If `.agents/product-marketing-context.md` exists (or `.claude/product-marketing-context.md` in older setups), read it before editing. Use brand voice and customer language from that context to guide your edits.
 
 Good copy editing isn't about rewriting—it's about enhancing. Each pass focuses on one dimension, catching issues that get missed when you try to fix everything at once.