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/llama-cpp/references/optimization.md

@@ -0,0 +1,89 @@
+# Performance Optimization Guide
+
+Maximize llama.cpp inference speed and efficiency.
+
+## CPU Optimization
+
+### Thread tuning
+```bash
+# Set threads (default: physical cores)
+./llama-cli -m model.gguf -t 8
+
+# For AMD Ryzen 9 7950X (16 cores, 32 threads)
+-t 16  # Best: physical cores
+
+# Avoid hyperthreading (slower for matrix ops)
+```
+
+### BLAS acceleration
+```bash
+# OpenBLAS (faster matrix ops)
+make LLAMA_OPENBLAS=1
+
+# BLAS gives 2-3× speedup
+```
+
+## GPU Offloading
+
+### Layer offloading
+```bash
+# Offload 35 layers to GPU (hybrid mode)
+./llama-cli -m model.gguf -ngl 35
+
+# Offload all layers
+./llama-cli -m model.gguf -ngl 999
+
+# Find optimal value:
+# Start with -ngl 999
+# If OOM, reduce by 5 until fits
+```
+
+### Memory usage
+```bash
+# Check VRAM usage
+nvidia-smi dmon
+
+# Reduce context if needed
+./llama-cli -m model.gguf -c 2048  # 2K context instead of 4K
+```
+
+## Batch Processing
+
+```bash
+# Increase batch size for throughput
+./llama-cli -m model.gguf -b 512  # Default: 512
+
+# Physical batch (GPU)
+--ubatch 128  # Process 128 tokens at once
+```
+
+## Context Management
+
+```bash
+# Default context (512 tokens)
+-c 512
+
+# Longer context (slower, more memory)
+-c 4096
+
+# Very long context (if model supports)
+-c 32768
+```
+
+## Benchmarks
+
+### CPU Performance (Llama 2-7B Q4_K_M)
+
+| Setup | Speed | Notes |
+|-------|-------|-------|
+| Apple M3 Max | 50 tok/s | Metal acceleration |
+| AMD 7950X (16c) | 35 tok/s | OpenBLAS |
+| Intel i9-13900K | 30 tok/s | AVX2 |
+
+### GPU Offloading (RTX 4090)
+
+| Layers GPU | Speed | VRAM |
+|------------|-------|------|
+| 0 (CPU only) | 30 tok/s | 0 GB |
+| 20 (hybrid) | 80 tok/s | 8 GB |
+| 35 (all) | 120 tok/s | 12 GB |