docs: update ONBOARDING and PLAYBOOK for current structure

ONBOARDING: add chapter list and difficulty tiers
PLAYBOOK: fix playbook reference (05→07), add CLI export pipeline,
use git switch instead of git checkout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

docs/ONBOARDING.md

@@ -52,12 +52,22 @@ is not yet wired to `chapters/`, so content may be placeholder.
 ## 5. Project context
 
 tutorial-git is part of the **Code with Branko** tutorial platform. It
-teaches Git from scratch — aimed at beginners with no prior experience.
+teaches Git across three difficulty tiers — Beginner (chapters 1-2),
+Advanced (chapters 3-5), and Expert (chapter 6). Chapters 7-9 are
+reference material (playbook, appendix, glossary).
 
 The content lives in `chapters/` as plain Markdown, independent of any
 static site generator. The Astro site in `astro-site/` consumes these
 files for the web version.
 
+```
+01-introduction.md         06-expert-topics.md
+02-building-blocks.md      07-playbook.md
+03-branching-and-merging.md 08-appendix.md
+04-remote-repositories.md  09-glossary.md
+05-subprojects.md
+```
+
 Chapter structure, writing conventions, and figure style rules are
 documented in `CLAUDE.md`. Quality conventions (git workflow, docs,
 code style) are inherited from the `docs/solid-ai-templates/` submodule.

docs/PLAYBOOK.md

@@ -33,7 +33,7 @@ Operational reference for common tasks in tutorial-git.
 - Place exercises at the end of the chapter under `## Exercises`
 - Each exercise has: **Task**, **Steps** (numbered), **Verify**
 - Exercises within a chapter use the same lab repository
-- Cross-chapter exercises go in `05-playbook.md`
+- Cross-topic recipes go in `07-playbook.md`
 
 ### 1.4 Adding quiz questions
 
@@ -55,6 +55,16 @@ Operational reference for common tasks in tutorial-git.
 
 ### 2.2 Exporting as PNG
 
+**CLI (recommended for batch export):**
+
+```bash
+DRAWIO="/c/Program Files/draw.io/draw.io.exe"
+"$DRAWIO" --export --format png --scale 2 --transparent \
+  --output assets/images/<name>.png assets/drawio/<name>.drawio
+```
+
+**Manual (single diagram):**
+
 1. Open the `.drawio` file in draw.io
 2. File → Export as → PNG
 3. Settings: transparent background, 2x scale
@@ -105,8 +115,8 @@ automatically.
 ### 4.1 Branch and commit
 
 ```bash
-git checkout main && git pull
-git checkout -b docs/chapter-name
+git switch main && git pull
+git switch -c docs/chapter-name
 # ... make changes ...
 git add chapters/NN-slug.md
 git commit -m "docs: description of change"
@@ -122,7 +132,7 @@ git push -u origin docs/chapter-name
 ### 4.3 After merge
 
 ```bash
-git checkout main && git pull
+git switch main && git pull
 git branch -d docs/chapter-name
 git push origin --delete docs/chapter-name
 ```