fix: restore all removed bundled skills + fix skills sync system

- Restored 21 skills removed in commits 757d012 and 740dd92:
  accelerate, audiocraft, code-review, faiss, flash-attention, gguf,
  grpo-rl-training, guidance, llava, nemo-curator, obliteratus, peft,
  pytorch-fsdp, pytorch-lightning, simpo, slime, stable-diffusion,
  tensorrt-llm, torchtitan, trl-fine-tuning, whisper

- Rewrote sync_skills() with proper update semantics:
  * New skills (not in manifest): copied to user dir
  * Existing skills (in manifest + on disk): updated via hash comparison
  * User-deleted skills (in manifest, not on disk): respected, not re-added
  * Stale manifest entries (removed from bundled): cleaned from manifest

- Added sync_skills() to CLI startup (cmd_chat) and gateway startup
  (start_gateway) — previously only ran during 'hermes update'

- Updated cmd_update output to show new/updated/cleaned counts

- Rewrote tests: 20 tests covering manifest CRUD, dir hashing, fresh
  install, user deletion respect, update detection, stale cleanup, and
  name collision handling

75 bundled skills total. 2002 tests pass.

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

@@ -0,0 +1,87 @@
+# 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