feat(labs): make every lab a self-contained, skip-friendly starting point

Each lab now stands on its own; no hard dependency on prior labs.
- App-based labs get a canonical tasks-app snapshot in lab/start/ (three
  baselines: v0 add/list/done; v1 +count; v2 +count/delete), assigned by where
  each module sits in the command timeline. Modules with a purpose-built app
  (M10 trap, M13 planted bug, M21) snapshot their own app; planted devices kept.
- Self-contained labs (M15/17/18/19/22/23/24/25/27, which operate on their own
  lab files) get a preamble pointing at modules/NN/lab/.
- Every module + capstone gets a "Starting point (skip-friendly)" preamble:
  copy the snapshot, git init -b main, commit -> clean status, then start.

Lets a learner skip around or recover: copy start/, commit, go. All snapshots
run; tools/check.sh passes; no em-dashes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfzV5QvtPDz8LJS3Pu5VLT

capstone/README.md

@@ -127,6 +127,17 @@ swappable part; the workflow is the durable skill*), and you just lived it inste
 
 ## Hands-on lab
 
+
+> **Starting point (this lab is skip-friendly).** The capstone runs the whole loop on one feature.
+> To begin from a clean app, copy the snapshot into a fresh `tasks-app` and make the first commit:
+>
+> ```bash
+> mkdir -p ~/ai-workflow-course/tasks-app
+> cp -r ~/ai-workflow-course/capstone/lab/start/. ~/ai-workflow-course/tasks-app/
+> cd ~/ai-workflow-course/tasks-app && git init -b main && git add -A && git commit -m "start: capstone"
+> ```
+>
+> Already carrying your `tasks-app` from earlier modules? Keep using it and ignore this box.
 **Lab language:** shell + Python, on the `tasks-app` repo. You'll direct Claude Code (`claude`; sub
 your own agent) to do the git and the edits (M4); you make the calls and verify each result.