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-GUIDE.md

@@ -5,6 +5,10 @@ running feature with its own list, detail, form, settings page, and report. It i
 the companion to [PLUGIN-QUICKSTART.md](PLUGIN-QUICKSTART.md): the quickstart gets
 you moving with `flask plugin new`; this guide explains *why* each piece looks the
 way it does by walking the shipped code of the exemplar plugin.
+> **Windows / VS Code:** command examples 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).
 
 `measuringtools` was chosen as the exemplar on purpose. It is the first plugin
 built after the framework matured (ADR-005 scoped it; ADR-008 changed how plugin