Add Cortex to the per-agent install examples

The skills CLI already supports --agent cortex (installs to
.cortex/skills/). Requested by the ISV partner team; command verified
end-to-end against the live catalog.

Signed-off-by: Moshe Abramovitch <moshea@nvidia.com>

Author: mosheabr

README.md

@@ -57,6 +57,12 @@ npx skills add nvidia/skills --skill cuopt-numerical-optimization-api --agent cl
 npx skills add nvidia/skills --skill cuopt-numerical-optimization-api --agent codex
 ```
 
+**Cortex**
+
+```bash
+npx skills add nvidia/skills --skill cuopt-numerical-optimization-api --agent cortex
+```
+
 **Cursor**
 
 ```bash