refactor: reorganize skills into sub-categories

The skills directory was getting disorganized — mlops alone had 40
skills in a flat list, and 12 categories were singletons with just
one skill each.

Code change:
- prompt_builder.py: Support sub-categories in skill scanner.
  skills/mlops/training/axolotl/SKILL.md now shows as category
  'mlops/training' instead of just 'mlops'. Backwards-compatible
  with existing flat structure.

Split mlops (40 skills) into 7 sub-categories:
- mlops/training (12): accelerate, axolotl, flash-attention,
  grpo-rl-training, peft, pytorch-fsdp, pytorch-lightning,
  simpo, slime, torchtitan, trl-fine-tuning, unsloth
- mlops/inference (8): gguf, guidance, instructor, llama-cpp,
  obliteratus, outlines, tensorrt-llm, vllm
- mlops/models (6): audiocraft, clip, llava, segment-anything,
  stable-diffusion, whisper
- mlops/vector-databases (4): chroma, faiss, pinecone, qdrant
- mlops/evaluation (5): huggingface-tokenizers,
  lm-evaluation-harness, nemo-curator, saelens, weights-and-biases
- mlops/cloud (2): lambda-labs, modal
- mlops/research (1): dspy

Merged singleton categories:
- gifs → media (gif-search joins youtube-content)
- music-creation → media (heartmula, songsee)
- diagramming → creative (excalidraw joins ascii-art)
- ocr-and-documents → productivity
- domain → research (domain-intel)
- feeds → research (blogwatcher)
- market-data → research (polymarket)

Fixed misplaced skills:
- mlops/code-review → software-development (not ML-specific)
- mlops/ml-paper-writing → research (academic writing)

Added DESCRIPTION.md files for all new/updated categories.

skills/mlops/evaluation/nemo-curator/references/deduplication.md

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+# Deduplication Guide
+
+Complete guide to exact, fuzzy, and semantic deduplication.
+
+## Exact deduplication
+
+Remove documents with identical content.
+
+```python
+from nemo_curator.modules import ExactDuplicates
+
+# Exact deduplication
+exact_dedup = ExactDuplicates(
+    id_field="id",
+    text_field="text",
+    hash_method="md5"  # or "sha256"
+)
+
+deduped = exact_dedup(dataset)
+```
+
+**Performance**: ~16× faster on GPU vs CPU
+
+## Fuzzy deduplication
+
+Remove near-duplicate documents using MinHash + LSH.
+
+```python
+from nemo_curator.modules import FuzzyDuplicates
+
+fuzzy_dedup = FuzzyDuplicates(
+    id_field="id",
+    text_field="text",
+    num_hashes=260,        # MinHash permutations (more = accurate)
+    num_buckets=20,        # LSH buckets (more = faster, less recall)
+    hash_method="md5",
+    jaccard_threshold=0.8  # Similarity threshold
+)
+
+deduped = fuzzy_dedup(dataset)
+```
+
+**Parameters**:
+- `num_hashes`: 128-512 (default 260)
+- `num_buckets`: 10-50 (default 20)
+- `jaccard_threshold`: 0.7-0.9 (default 0.8)
+
+**Performance**: 16× faster on 8TB dataset (120h → 7.5h)
+
+## Semantic deduplication
+
+Remove semantically similar documents using embeddings.
+
+```python
+from nemo_curator.modules import SemanticDuplicates
+
+semantic_dedup = SemanticDuplicates(
+    id_field="id",
+    text_field="text",
+    embedding_model="sentence-transformers/all-MiniLM-L6-v2",
+    embedding_batch_size=256,
+    threshold=0.85,  # Cosine similarity threshold
+    device="cuda"
+)
+
+deduped = semantic_dedup(dataset)
+```
+
+**Models**:
+- `all-MiniLM-L6-v2`: Fast, 384 dims
+- `all-mpnet-base-v2`: Better quality, 768 dims
+- Custom models supported
+
+## Comparison
+
+| Method | Speed | Recall | Use Case |
+|--------|-------|--------|----------|
+| Exact | Fastest | 100% | Exact matches only |
+| Fuzzy | Fast | ~95% | Near-duplicates (recommended) |
+| Semantic | Slow | ~90% | Paraphrases, rewrites |
+
+## Best practices
+
+1. **Start with exact dedup** - Remove obvious duplicates
+2. **Use fuzzy for large datasets** - Best speed/quality trade-off
+3. **Semantic for high-value data** - Expensive but thorough
+4. **GPU acceleration required** - 10-16× speedup

skills/mlops/evaluation/nemo-curator/references/filtering.md

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+# Quality Filtering Guide
+
+Complete guide to NeMo Curator's 30+ quality filters.
+
+## Text-based filters
+
+### Word count
+
+```python
+from nemo_curator.filters import WordCountFilter
+
+# Filter by word count
+dataset = dataset.filter(WordCountFilter(min_words=50, max_words=100000))
+```
+
+### Repeated content
+
+```python
+from nemo_curator.filters import RepeatedLinesFilter
+
+# Remove documents with >30% repeated lines
+dataset = dataset.filter(RepeatedLinesFilter(max_repeated_line_fraction=0.3))
+```
+
+### Symbol ratio
+
+```python
+from nemo_curator.filters import SymbolToWordRatioFilter
+
+# Remove documents with too many symbols
+dataset = dataset.filter(SymbolToWordRatioFilter(max_symbol_to_word_ratio=0.3))
+```
+
+### URL ratio
+
+```python
+from nemo_curator.filters import UrlRatioFilter
+
+# Remove documents with many URLs
+dataset = dataset.filter(UrlRatioFilter(max_url_ratio=0.2))
+```
+
+## Language filtering
+
+```python
+from nemo_curator.filters import LanguageIdentificationFilter
+
+# Keep only English documents
+dataset = dataset.filter(LanguageIdentificationFilter(target_languages=["en"]))
+
+# Multiple languages
+dataset = dataset.filter(LanguageIdentificationFilter(target_languages=["en", "es", "fr"]))
+```
+
+## Classifier-based filtering
+
+### Quality classifier
+
+```python
+from nemo_curator.classifiers import QualityClassifier
+
+quality_clf = QualityClassifier(
+    model_path="nvidia/quality-classifier-deberta",
+    batch_size=256,
+    device="cuda"
+)
+
+# Filter low-quality (threshold > 0.5 = high quality)
+dataset = dataset.filter(lambda doc: quality_clf(doc["text"]) > 0.5)
+```
+
+### NSFW classifier
+
+```python
+from nemo_curator.classifiers import NSFWClassifier
+
+nsfw_clf = NSFWClassifier(threshold=0.9, device="cuda")
+
+# Remove NSFW content
+dataset = dataset.filter(lambda doc: nsfw_clf(doc["text"]) < 0.9)
+```
+
+## Heuristic filters
+
+Full list of 30+ filters:
+- WordCountFilter
+- RepeatedLinesFilter
+- UrlRatioFilter
+- SymbolToWordRatioFilter
+- NonAlphaNumericFilter
+- BulletsFilter
+- WhiteSpaceFilter
+- ParenthesesFilter
+- LongWordFilter
+- And 20+ more...
+
+## Best practices
+
+1. **Apply cheap filters first** - Word count before GPU classifiers
+2. **Tune thresholds on sample** - Test on 10k docs before full run
+3. **Use GPU classifiers sparingly** - Expensive but effective
+4. **Chain filters efficiently** - Order by cost (cheap → expensive)