docs: define hf skill guidance for plugin install

plans/harborforge-cli-go-plan.md

@@ -167,6 +167,22 @@ In other words:
 - normal plugin install copies regular plugin skills
 - `hf` skill content is gated behind `--install-cli`
 
+#### `skills/hf/SKILL.md` content requirement
+
+`skills/hf/` should contain a `SKILL.md` file.
+
+That skill file should:
+- introduce the basic usage of `hf`
+- explain that `hf` is the HarborForge CLI
+- encourage agents to use:
+
+```bash
+hf --help-brief
+```
+
+when they want a concise view of the currently permitted command surface
+- point agents to normal `--help` when they need the full command tree including `(not permitted)` entries
+
 ### 8. Implication for implementation planning
 
 This means the CLI plan is no longer only about `hf` command syntax.