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/DEVELOPMENT-SETUP.md

@@ -182,12 +182,12 @@ tasks call `venv/` and `frontend/node_modules`).
    bash scripts/check-naming-and-style.sh      # naming - runs via Git Bash
    ```
    There is NO auto-installed git hook - you run these yourself (or the
-   VS Code task). On the internal server, CI runs all three on every push
-   and fails the build on a bad name. GitHub Actions is not set up on the
-   public mirror yet, so if you develop against GitHub these local checks
-   (or the opt-in hook below) are your only gate - run them. The naming
-   check is a shell script, so that one line needs Git Bash (installed with
-   Git for Windows).
+   VS Code task). CI runs all three on every push and pull request
+   (`.github/workflows/ci.yml` on GitHub Actions; the same gate runs on the
+   internal server) and fails the build on a bad name, so nothing bad
+   reaches `main` - running them locally just saves the round trip. The
+   naming check is a shell script, so that one line needs Git Bash
+   (installed with Git for Windows).
 
    Want it automatic? The repo ships a hook; enable it once per clone:
    ```powershell