chore: update context7 submodule to skill-based version

Update context7 plugin to use Skill system instead of SessionStart hook.

The new version provides intelligent, on-demand activation:
- Claude loads Context7 only when working with libraries/frameworks
- Significantly reduces token usage
- Simpler maintenance with single SKILL.md file

Upstream PR: upstash/context7#786
Commit: 13f4296

Author: amondnet