add marp skill and slide split plan

PLAN-split-slides.md

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+# Plan: Split index.md into multiple files
+
+## Current state
+- ~4200 lines, 5 sessions + intro/outro
+- Shared: frontmatter, styles, intro slides (title, about, schedule, survey, overview)
+- Per-session: actual content
+
+## Proposed structure
+
+```
+slides/
+├── _frontmatter.yml      # shared marp config
+├── _styles.md            # shared CSS
+├── _intro.md             # title, about, schedule, survey, overview
+├── termin-1.md           # Session 1 content
+├── termin-2.md           # Session 2 content
+├── termin-3.md           # Session 3 content
+├── termin-4.md           # Session 4 content
+├── termin-5.md           # TBA session
+└── _outro.md             # Q&A, license
+```
+
+## Makefile targets
+
+```makefile
+# Shared parts
+HEADER = slides/_frontmatter.yml slides/_styles.md
+INTRO  = slides/_intro.md
+OUTRO  = slides/_outro.md
+
+# Build single termin (intro + content + outro)
+build-termin-%:
+	cat $(HEADER) $(INTRO) slides/termin-$*.md $(OUTRO) > build/termin-$*.md
+	marp build/termin-$*.md -o build/termin-$*.html
+
+# Dev mode for single termin
+dev-%:
+	cat $(HEADER) $(INTRO) slides/termin-$*.md $(OUTRO) > build/.dev.md
+	PORT=1312 marp --server build/
+
+# Build all
+build-all:
+	cat $(HEADER) $(INTRO) slides/termin-*.md $(OUTRO) > build/all.md
+	marp build/all.md -o build/all.html
+```
+
+## Usage
+
+```bash
+make dev-1          # preview termin 1
+make dev-3          # preview termin 3
+make build-termin-2 # build termin 2
+make build-all      # build everything
+```
+
+## Trade-offs
+- Requires intermediate file generation
+- Can't use `marp --watch` directly on source (need wrapper)
+- But: clean separation, single source of truth for styles
+
+## Status
+- [ ] Learn Marp/Marpit in depth first
+- [ ] Implement split
+- [ ] Update Makefile
+- [ ] Test all targets