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

modules/13-testing-in-the-ai-era/lab/start/README.md

@@ -0,0 +1,25 @@
+# Demo app: `tasks` (Module 13 copy)
+
+The same tiny task tracker from Modules 1 and 2, with one feature added: a `count` command backed
+by `TaskList.pending_count()`. Use this copy for the Module 13 lab so everyone starts from the same
+code, including the same latent bug.
+
+If you already have a `tasks-app` from earlier modules, you can use that instead; just make sure it
+has a `count` command (the Module 2 lab added one). The planted bug in this copy is there on purpose.
+
+## Files
+
+- `tasks.py`: core logic (`Task`, `TaskList`), now with `pending_count()`.
+- `cli.py`: command-line front end. Adds `count`.
+
+## Run it
+
+```bash
+python cli.py add "write the tests"
+python cli.py add "fix the bug"
+python cli.py done 0
+python cli.py list
+python cli.py count
+```
+
+Requires Python 3.10+. No third-party packages; tests use the standard library `unittest`.