docs: update Copilot CLI install to use fetch pattern

Matches the OpenCode/Codex pattern of telling the agent to fetch
and follow the INSTALL.md instructions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mhenke

README.md

@@ -81,8 +81,10 @@ Fetch and follow instructions from https://raw.githubusercontent.com/obra/superp
 
 ### GitHub Copilot CLI
 
-```bash
-git clone https://github.com/obra/superpowers.git ~/.copilot/superpowers && ~/.copilot/superpowers/.copilot/install.sh
+Tell Copilot CLI:
+
+```
+Fetch and follow instructions from https://raw.githubusercontent.com/obra/superpowers/refs/heads/main/.copilot/INSTALL.md
 ```
 
 **Detailed docs:** [docs/README.copilot.md](docs/README.copilot.md)