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-EXTERNAL-REPO.md

@@ -7,6 +7,12 @@ the framework ships a bundled set, and you drop your own plugin into
 `<framework>/plugins/<name>/` by clone, submodule, or symlink. No pip packaging
 is required for v1 (pip distribution is deferred to v2 per ADR-003).
 
+> **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).
+
+
 If you have not written a plugin before, start with
 [PLUGIN-QUICKSTART.md](PLUGIN-QUICKSTART.md) and the hook reference in
 [PLUGIN-HOOKS.md](PLUGIN-HOOKS.md). This document only covers the parts that are