refactor(skills): align MPI skills with Agent Skills best practices

The skill metadata had drifted: every SKILL.md name field lacked the
mpi- prefix, contradicting the directory names and the Agent Skills
specification. Descriptions were also missing negative triggers, making
it easy for the agent to load the wrong skill.

Rewrote the pdf-to-docx skill to follow progressive disclosure: the main
SKILL.md dropped from 318 lines to 80, with detailed code examples moved
to on-demand references. Added uv run instructions and /// script PEP 723
metadata so dependencies are declared inline and installed automatically.
Fixed the pptx skill's script paths and added CLI usage messages to both
pptx scripts and the normalize script.

Removed the empty self-review directory that was superseded by the unified
translation-review skill.

skills/readme.dj

@@ -14,6 +14,20 @@ skills:
 
 {% Replace `/path/to/mpi` with the absolute path to this repository. Edit config.yaml directly — `hermes config set` stores list values as strings. %}
 
+## Structure
+
+Each skill follows the Agent Skills layout:
+
+```
+skill-name/
+├── SKILL.md          # frontmatter + core instructions
+├── scripts/          # tiny deterministic CLIs
+├── references/       # docs loaded on demand
+└── assets/           # templates and static files
+```
+
+The `name` field in each `SKILL.md` frontmatter matches the directory name.
+
 ## Skills
 
 | Name | What it does |