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/inference/instructor/references/examples.md

@@ -0,0 +1,107 @@
+# Real-World Examples
+
+Practical examples of using Instructor for structured data extraction.
+
+## Data Extraction
+
+```python
+class CompanyInfo(BaseModel):
+    name: str
+    founded: int
+    industry: str
+    employees: int
+
+text = "Apple was founded in 1976 in the technology industry with 164,000 employees."
+
+company = client.messages.create(
+    model="claude-sonnet-4-5-20250929",
+    max_tokens=1024,
+    messages=[{"role": "user", "content": f"Extract: {text}"}],
+    response_model=CompanyInfo
+)
+```
+
+## Classification
+
+```python
+class Sentiment(str, Enum):
+    POSITIVE = "positive"
+    NEGATIVE = "negative"
+    NEUTRAL = "neutral"
+
+class Review(BaseModel):
+    sentiment: Sentiment
+    confidence: float = Field(ge=0.0, le=1.0)
+
+review = client.messages.create(
+    model="claude-sonnet-4-5-20250929",
+    max_tokens=1024,
+    messages=[{"role": "user", "content": "This product is amazing!"}],
+    response_model=Review
+)
+```
+
+## Multi-Entity Extraction
+
+```python
+class Person(BaseModel):
+    name: str
+    role: str
+
+class Entities(BaseModel):
+    people: list[Person]
+    organizations: list[str]
+    locations: list[str]
+
+entities = client.messages.create(
+    model="claude-sonnet-4-5-20250929",
+    max_tokens=1024,
+    messages=[{"role": "user", "content": "Tim Cook, CEO of Apple, spoke in Cupertino..."}],
+    response_model=Entities
+)
+```
+
+## Structured Analysis
+
+```python
+class Analysis(BaseModel):
+    summary: str
+    key_points: list[str]
+    sentiment: Sentiment
+    actionable_items: list[str]
+
+analysis = client.messages.create(
+    model="claude-sonnet-4-5-20250929",
+    max_tokens=1024,
+    messages=[{"role": "user", "content": "Analyze: [long text]"}],
+    response_model=Analysis
+)
+```
+
+## Batch Processing
+
+```python
+texts = ["text1", "text2", "text3"]
+results = [
+    client.messages.create(
+        model="claude-sonnet-4-5-20250929",
+        max_tokens=1024,
+        messages=[{"role": "user", "content": text}],
+        response_model=YourModel
+    )
+    for text in texts
+]
+```
+
+## Streaming
+
+```python
+for partial in client.messages.create_partial(
+    model="claude-sonnet-4-5-20250929",
+    max_tokens=1024,
+    messages=[{"role": "user", "content": "Generate report..."}],
+    response_model=Report
+):
+    print(f"Progress: {partial.title}")
+    # Update UI in real-time
+```