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/acl_lualatex.tex

@@ -1,101 +0,0 @@
-% This file compiles with both LuaLaTeX and XeLaTeX
-\documentclass[11pt]{article}
-
-% Change "review" to "final" to generate the final (sometimes called camera-ready) version.
-% Change to "preprint" to generate a non-anonymous version with page numbers.
-\usepackage[review]{acl}
-
-% This is not strictly necessary, and may be commented out,
-% but it will improve the layout of the manuscript,
-% and will typically save some space.
- \usepackage{microtype}
-
-% If the title and author information does not fit in the area allocated, uncomment the following
-%
-%\setlength\titlebox{<dim>}
-%
-% and set <dim> to something 5cm or larger.
-
-% These font selection commands work with
-% LuaLaTeX and XeLaTeX, but not pdfLaTeX.
-\usepackage[english,bidi=default]{babel} % English as the main language.
-\babelfont{rm}{TeXGyreTermesX} % similar to Times
-%%% include whatever languages you need below this line
-\babelprovide[import]{hindi}
-\babelfont[*devanagari]{rm}{Lohit Devanagari}
-\babelprovide[import]{arabic}
-\babelfont[*arabic]{rm}{Noto Sans Arabic}
-
-
-%\usepackage{polyglossia}
-%\setdefaultlanguage{english}
-%\setotherlanguages{arabic,russian,thai,hindi,kannada}
-
-%%%%%
-
-
-\title{LuaLaTeX and XeLaTeX Template for *ACL Style Files}
-
-% Author information can be set in various styles:
-% For several authors from the same institution:
-% \author{Author 1 \and ... \and Author n \\
-%         Address line \\ ... \\ Address line}
-% if the names do not fit well on one line use
-%         Author 1 \\ {\bf Author 2} \\ ... \\ {\bf Author n} \\
-% For authors from different institutions:
-% \author{Author 1 \\ Address line \\  ... \\ Address line
-%         \And  ... \And
-%         Author n \\ Address line \\ ... \\ Address line}
-% To start a seperate ``row'' of authors use \AND, as in
-% \author{Author 1 \\ Address line \\  ... \\ Address line
-%         \AND
-%         Author 2 \\ Address line \\ ... \\ Address line \And
-%         Author 3 \\ Address line \\ ... \\ Address line}
-
-\author{First Author \\
-  Affiliation / Address line 1 \\
-  Affiliation / Address line 2 \\
-  Affiliation / Address line 3 \\
-  \texttt{email@domain} \\\And
-  Second Author \\
-  Affiliation / Address line 1 \\
-  Affiliation / Address line 2 \\
-  Affiliation / Address line 3 \\
-  \texttt{email@domain} \\}
-
-\begin{document}
-
-\maketitle
-\begin{abstract}
-This document provides an example showing how
-to use the *ACL style files with either
-LuaLaTeX or XeLaTeX.
-\end{abstract}
-
-
-\section{Introduction}
-
-Please see the general instructions
-in the file \verb|acl_latex.tex|.
-
-Here are some examples of text in various languages.
-
-Hindi: \foreignlanguage{hindi}{मानव अधिकारों की सार्वभौम घोषणा}
-
-Arabic: \foreignlanguage{arabic}{الإعلان العالمي لحقوق الإنسان}
-
-Here is an example citation:
-\citet{Gusfield:97} argues that...
-
-
-% Entries for the entire Anthology, followed by custom entries
-\bibliography{custom}
-
-\appendix
-
-\section{Example Appendix}
-\label{sec:appendix}
-
-This is an appendix.
-
-\end{document}