Add GitHub Actions CI + Windows notes on the developer docs

GitHub had no CI, so naming/tests/build were unenforced on the public
mirror. Add .github/workflows/ci.yml mirroring the internal pipeline:
backend pytest, the naming gate, frontend vitest+build, and the
migrations-mysql job that proves a fresh flask db upgrade + every
plugin chain on utf8mb4 MySQL 8 is idempotent. Flip the dev-setup CI
note to reflect it. Add an identical Windows/VS Code convention note to
the four developer docs (venv\Scripts vs venv/bin, $env: vs export,
pointer to DEVELOPMENT-SETUP).

docs/PLUGIN-QUICKSTART.md

@@ -3,6 +3,12 @@
 Build a working shopdb-flask plugin in 30 minutes. This walks through generating, customizing, installing, and testing a plugin from scratch.
 
 For the full hook reference, see [PLUGIN-HOOKS.md](PLUGIN-HOOKS.md).
+
+> **Windows / VS Code:** command examples below use the Linux venv path
+> `venv/bin/python`; on Windows use `venv\Scripts\python` and
+> `$env:FLASK_APP="shopdb"` (not `export`). Full Windows onboarding:
+> [DEVELOPMENT-SETUP](DEVELOPMENT-SETUP).
+
 For the architectural decisions behind the contract, see [docs/adr/](../docs/adr/).
 
 ## Step 1: Generate the skeleton