feat(skills): categorize tap skills from skills.sh.json grouping sidecar

A GitHub tap can ship a repo-root skills.sh.json (the published skills.sh
schema) declaring category groupings. The Skills Hub now reads it at index
time and uses each grouping title as the skill's category label, instead of
the tag-derived guess. Generic: any tap that ships the file gets real
categorization — NVIDIA's groupings (Inference AI, Decision Optimization,
GPU Development, etc.) flow through automatically.

- GitHubSource: _get_skillsh_groupings() fetches+caches the sidecar per repo;
  _parse_skillsh_groupings() flattens it to {skill_name: title};
  _list_skills_in_repo() stamps meta.extra['category']; _meta_to_dict now
  serializes extra so the category survives the index cache round-trip.
- extract-skills.py: prefers extra['category'] over the tag heuristic and
  exempts sidecar categories from the small-category to Other collapse.
- Docs + 12 tests.

website/scripts/extract-skills.py

@@ -343,6 +343,15 @@ def extract_unified_index_skills():
         category = _guess_category(tags)
         extra = entry.get("extra", {}) or {}
 
+        # A skills.sh.json grouping sidecar (if the tap ships one) gives us a
+        # real, human-readable category — prefer it over the tag heuristic.
+        # extra["category"] holds the grouping title, e.g. "Inference AI".
+        sidecar_category = extra.get("category") if isinstance(extra, dict) else None
+        category_label_override = ""
+        if isinstance(sidecar_category, str) and sidecar_category.strip():
+            category_label_override = sidecar_category.strip()
+            category = category_label_override.lower().replace(" ", "-")
+
         # Author hint from extras when available (skills.sh has installs;
         # clawhub doesn't expose author).
         author = ""
@@ -358,7 +367,8 @@ def extract_unified_index_skills():
             "description": description,
             "overview": "",
             "category": category,
-            "categoryLabel": "",  # filled in _consolidate_small_categories
+            "categoryLabel": category_label_override,  # set from sidecar, else filled in _consolidate_small_categories
+            "fixedCategory": bool(category_label_override),  # sidecar categories are exempt from small-cat collapse
             "source": source_label,
             "tags": tags,
             "platforms": [],
@@ -491,10 +501,17 @@ def _consolidate_small_categories(skills: list) -> list:
             s["category"] = "other"
             s["categoryLabel"] = "Other"
 
-    counts = Counter(s["category"] for s in skills)
+    # Skills with a sidecar-declared category (skills.sh.json grouping) keep
+    # their category even if it's the only skill in it — the tap explicitly
+    # chose that label, so it's not a heuristic guess to collapse away.
+    counts = Counter(
+        s["category"] for s in skills if not s.get("fixedCategory")
+    )
     small_cats = {cat for cat, n in counts.items() if n < MIN_CATEGORY_SIZE}
 
     for s in skills:
+        if s.get("fixedCategory"):
+            continue
         if s["category"] in small_cats:
             s["category"] = "other"
             s["categoryLabel"] = "Other"