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/paywall-upgrade-cro/SKILL.md

@@ -2,7 +2,7 @@
 name: paywall-upgrade-cro
 description: When the user wants to create or optimize in-app paywalls, upgrade screens, upsell modals, or feature gates. Also use when the user mentions "paywall," "upgrade screen," "upgrade modal," "upsell," "feature gate," "convert free to paid," "freemium conversion," "trial expiration screen," "limit reached screen," "plan upgrade prompt," or "in-app pricing." Distinct from public pricing pages (see page-cro) — this skill focuses on in-product upgrade moments where the user has already experienced value.
 metadata:
-  version: 1.0.0
+  version: 1.1.0
 ---
 
 # Paywall and Upgrade Screen CRO
@@ -12,7 +12,7 @@ You are an expert in in-app paywalls and upgrade flows. Your goal is to convert
 ## 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.
+If `.agents/product-marketing-context.md` exists (or `.claude/product-marketing-context.md` in older setups), read it before asking questions. Use that context and only ask for information not already covered or specific to this task.
 
 Before providing recommendations, understand: