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/gifs/gif-search/SKILL.md

@@ -1,73 +0,0 @@
----
-name: gif-search
-description: Search and download GIFs from Tenor using curl. No dependencies beyond curl and jq. Useful for finding reaction GIFs, creating visual content, and sending GIFs in chat.
-version: 1.0.0
-author: Hermes Agent
-license: MIT
-metadata:
-  hermes:
-    tags: [GIF, Media, Search, Tenor, API]
----
-
-# GIF Search (Tenor API)
-
-Search and download GIFs directly via the Tenor API using curl. No extra tools needed.
-
-## Prerequisites
-
-- `curl` and `jq` (both standard on Linux)
-
-## Search for GIFs
-
-```bash
-# Search and get GIF URLs
-curl -s "https://tenor.googleapis.com/v2/search?q=thumbs+up&limit=5&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[].media_formats.gif.url'
-
-# Get smaller/preview versions
-curl -s "https://tenor.googleapis.com/v2/search?q=nice+work&limit=3&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[].media_formats.tinygif.url'
-```
-
-## Download a GIF
-
-```bash
-# Search and download the top result
-URL=$(curl -s "https://tenor.googleapis.com/v2/search?q=celebration&limit=1&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq -r '.results[0].media_formats.gif.url')
-curl -sL "$URL" -o celebration.gif
-```
-
-## Get Full Metadata
-
-```bash
-curl -s "https://tenor.googleapis.com/v2/search?q=cat&limit=3&key=AIzaSyAyimkuYQYF_FXVALexPuGQctUWRURdCYQ" | jq '.results[] | {title: .title, url: .media_formats.gif.url, preview: .media_formats.tinygif.url, dimensions: .media_formats.gif.dims}'
-```
-
-## API Parameters
-
-| Parameter | Description |
-|-----------|-------------|
-| `q` | Search query (URL-encode spaces as `+`) |
-| `limit` | Max results (1-50, default 20) |
-| `key` | API key (the one above is Tenor's public demo key) |
-| `media_filter` | Filter formats: `gif`, `tinygif`, `mp4`, `tinymp4`, `webm` |
-| `contentfilter` | Safety: `off`, `low`, `medium`, `high` |
-| `locale` | Language: `en_US`, `es`, `fr`, etc. |
-
-## Available Media Formats
-
-Each result has multiple formats under `.media_formats`:
-
-| Format | Use case |
-|--------|----------|
-| `gif` | Full quality GIF |
-| `tinygif` | Small preview GIF |
-| `mp4` | Video version (smaller file size) |
-| `tinymp4` | Small preview video |
-| `webm` | WebM video |
-| `nanogif` | Tiny thumbnail |
-
-## Notes
-
-- The API key above is Tenor's public demo key — it works but has rate limits
-- URL-encode the query: spaces as `+`, special chars as `%XX`
-- For sending in chat, `tinygif` URLs are lighter weight
-- GIF URLs can be used directly in markdown: `![alt](url)`