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/research/ml-paper-writing/templates/acl/README.md

@@ -0,0 +1,50 @@
+# *ACL Paper Styles
+
+This directory contains the latest LaTeX templates for *ACL conferences.
+
+## Instructions for authors
+
+Paper submissions to *ACL conferences must use the official ACL style
+templates.
+
+The LaTeX style files are available
+
+- as an [Overleaf template](https://www.overleaf.com/latex/templates/association-for-computational-linguistics-acl-conference/jvxskxpnznfj)
+- in this repository
+- as a [.zip file](https://github.com/acl-org/acl-style-files/archive/refs/heads/master.zip)
+
+Please see [`acl_latex.tex`](https://github.com/acl-org/acl-style-files/blob/master/acl_latex.tex) for an example.
+
+Please follow the paper formatting guidelines general to *ACL
+conferences:
+
+- [Paper formatting guidelines](https://acl-org.github.io/ACLPUB/formatting.html)
+
+Authors may not modify these style files or use templates designed for
+other conferences.
+
+## Instructions for publications chairs
+
+To adapt the style files for your conference, please fork this repository and
+make necessary changes. Minimally, you'll need to update the name of
+the conference and rename the files.
+
+If you make improvements to the templates that should be propagated to
+future conferences, please submit a pull request. Thank you in
+advance!
+
+In older versions of the templates, authors were asked to fill in the
+START submission ID so that it would be stamped at the top of each
+page of the anonymized version. This is no longer needed, because it
+is now possible to do this stamping automatically within
+START. Currently, the way to do this is for the program chair to email
+support@softconf.com and request it.
+
+## Instructions for making changes to style files
+
+- merge pull request in github, or push to github
+- git pull from github to a local repository
+- then, git push from your local repository to overleaf project 
+    - Overleaf project is https://www.overleaf.com/project/5f64f1fb97c4c50001b60549
+    - Overleaf git url is https://git.overleaf.com/5f64f1fb97c4c50001b60549
+- then, click "Submit" and then "Submit as Template" in overleaf in order to ask overleaf to update the overleaf template from the overleaf project