feat: NSIS Windows installer and update check (#32)

- windows/installer.nsi: installs to Program Files, manages system PATH,
  registers uninstaller
- relspec update [--check] [--yes]: compares against the latest release and
  prompts; on Windows downloads and runs the installer
- release workflow builds and uploads relspec-setup-windows-amd64.exe
- make installer-windows

windows/README.md

@@ -0,0 +1,29 @@
+# Windows installer
+
+NSIS script: `installer.nsi`. Output: `build/relspec-setup-windows-amd64.exe`.
+
+## Build
+
+```
+make installer-windows
+```
+
+Needs `makensis` (package `nsis`). CI builds and uploads it on release.
+
+## Installer
+
+- Installs `relspec.exe` to `%ProgramFiles%\RelSpec` (admin).
+- Adds the install dir to the system `PATH`; removed on uninstall.
+- Registers in Add/Remove Programs.
+
+## Update check
+
+```
+relspec update            # check, prompt, download + run installer (Windows)
+relspec update --check    # report only
+relspec 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+- `dev` and commit-hash builds are never reported as outdated.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.