Default KGet's download folder to Downloads and add release workflow

install.sh now sets KGet's remembered "New Download" folder
(kgetrc Internal/LastDirectory) to the XDG downloads directory if it
isn't already configured, since KGet's own out-of-the-box default was
Documents. Falls back correctly even when xdg-user-dir has no
user-dirs.dirs config to read (it silently returns bare $HOME
otherwise).

Also add a GitHub Actions workflow that packages extension/ into a zip
and publishes a GitHub Release with generated notes whenever a
v*.*.* tag is pushed.

README.md

@@ -51,6 +51,19 @@ This removes the native messaging host registrations and the installed host
 script. You'll still need to remove the unpacked extension yourself via
 `chrome://extensions` — that step can't be scripted.
 
+## Releases
+
+Pushing a tag matching `v*.*.*` (e.g. `v1.0.0`) triggers a GitHub Actions
+workflow ([.github/workflows/release.yml](.github/workflows/release.yml))
+that zips `extension/` and publishes it as a GitHub Release with
+auto-generated release notes. Bump `"version"` in `extension/manifest.json`
+before tagging so the packaged extension's version matches.
+
+```bash
+git tag v1.0.0
+git push origin v1.0.0
+```
+
 ## Known limitations
 
 - **`blob:`/`data:` URLs can't be captured.** There's no real network URL to