Add shell completion support

specs/versions/0.1/commands.md

@@ -49,6 +49,20 @@ gsp version
 gsp version --json
 ```
 
+## completion
+
+Print or install shell completion scripts.
+
+```bash
+gsp completion powershell
+gsp completion bash
+gsp completion zsh
+gsp completion fish
+gsp completion install powershell
+```
+
+Completion covers subcommands, common flags, enum values, and GSP ids from `gsp index --root .`.
+
 ## validate
 
 Validate GSP files and references.