Diffs: Migrate tool usage guidance from before_prompt_build to a plugin skill (#32630)

Merged via squash.

Prepared head SHA: 585697a4e1556baa2cd79a7b449b120c4fd87e17
Co-authored-by: sircrumpet <4436535+sircrumpet@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

extensions/diffs/openclaw.plugin.json

@@ -2,6 +2,7 @@
   "id": "diffs",
   "name": "Diffs",
   "description": "Read-only diff viewer and file renderer for agents.",
+  "skills": ["./skills"],
   "uiHints": {
     "defaults.fontFamily": {
       "label": "Default Font",