[fix] Cursor npx install path docs

[charliec05]

Summary

• Document that npx skills add ... --global installs the Cursor skill into the universal ~/.agents/skills/cortex-code/ path.
• Keep the local integrations/cursor/install.sh path documented separately as ~/.cursor/skills/cortex-code/.
• Add a Cursor integration regression test so the docs keep copying the routing rule from the correct npx install path.

Fixes #15.

Validation

• .venv/bin/python -m pytest tests/integrations/cursor/test_install.py -q
• git diff --check