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/feeds/blogwatcher/SKILL.md

@@ -1,54 +0,0 @@
----
-name: blogwatcher
-description: Monitor blogs and RSS/Atom feeds for updates using the blogwatcher CLI. Add blogs, scan for new articles, and track what you've read.
-version: 1.0.0
-author: community
-license: MIT
-metadata:
-  hermes:
-    tags: [RSS, Blogs, Feed-Reader, Monitoring]
-    homepage: https://github.com/Hyaxia/blogwatcher
----
-
-# Blogwatcher
-
-Track blog and RSS/Atom feed updates with the `blogwatcher` CLI.
-
-## Prerequisites
-
-- Go installed (`go version` to check)
-- Install: `go install github.com/Hyaxia/blogwatcher/cmd/blogwatcher@latest`
-
-## Common Commands
-
-- Add a blog: `blogwatcher add "My Blog" https://example.com`
-- List blogs: `blogwatcher blogs`
-- Scan for updates: `blogwatcher scan`
-- List articles: `blogwatcher articles`
-- Mark an article read: `blogwatcher read 1`
-- Mark all articles read: `blogwatcher read-all`
-- Remove a blog: `blogwatcher remove "My Blog"`
-
-## Example Output
-
-```
-$ blogwatcher blogs
-Tracked blogs (1):
-
-  xkcd
-    URL: https://xkcd.com
-```
-
-```
-$ blogwatcher scan
-Scanning 1 blog(s)...
-
-  xkcd
-    Source: RSS | Found: 4 | New: 4
-
-Found 4 new article(s) total!
-```
-
-## Notes
-
-- Use `blogwatcher <command> --help` to discover flags and options.