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/ml-paper-writing/templates/acl/anthology.bib.txt

@@ -1,26 +0,0 @@
-For citing papers in the ACL Anthology, we provide a single consolidated
-BibTeX file containing all of its papers. The bibkeys in these papers are
-designed to be semantic in nature: {names}-{year}-{words}, where
-- `names` is the concatenated last names of the authors when there is just
-  one or two authors, or `lastname-etal` for 3+
-- `year` is the four-digit year
-- `words` is the first significant word in the title, or more, if necessary,
-  to preserve uniqueness
-
-For example, https://aclanthology.org/N04-1035 can be cited as \cite{galley-etal-2004-whats}.
-
-The consolidated file can be downloaded from here:
-- https://aclanthology.org/anthology.bib
-
-Unfortunately, as of 2024 or so, this file is now larger than 50 MB, which is Overleaf's
-bib file size limit. Consequently, the Anthology shards the file automatically into
-49 MB shards.
-
-There are currently (2025) two files:
-- https://aclanthology.org/anthology-1.bib
-- https://aclanthology.org/anthology-2.bib
-
-You can download these directly from Overleaf from New File -> From External URL,
-and then adding them to the \bibliography line in acl_latex.tex:
-
-    \bibliography{custom,anthology-1,anthology-2}