feat(windows): rewrite NSIS installer, add pgtidy update command

windows/README.md

@@ -0,0 +1,29 @@
+# Windows installer
+
+NSIS script: `installer.nsi`. Output: `dist/pgtidy-setup-windows-amd64.exe`.
+
+## Build
+
+```
+make installer-windows
+```
+
+Needs `makensis` (package `nsis`). CI builds and uploads it on release.
+
+## Installer
+
+- Installs `pgtidy.exe` to `%ProgramFiles%\PgTidy` (admin).
+- Adds the install dir to the system `PATH`; removed on uninstall.
+- Registers in Add/Remove Programs (supports silent `/S` uninstall).
+
+## Update check
+
+```
+pgtidy update            # check, prompt, download + run installer (Windows)
+pgtidy update --check    # report only
+pgtidy update --yes      # no prompt
+```
+
+- Source: latest Gitea release (`pkg/updatecheck`).
+- `dev` and commit-hash builds are never reported as outdated.
+- Non-Windows: prints the release URL.