Add README, plugin citations, and multi-platform release CI.

Ship screenshots and third-party notices for the desktop shell, and
package Windows, macOS, deb, rpm, and Flatpak from GitHub Releases.

Co-authored-by: Cursor <cursoragent@cursor.com>

bin/dsh-desktop

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Run a built binary from the checkout, or fall back to cargo.
+set -e
+ROOT=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+for candidate in \
+  "$ROOT/target/release/dsh-desktop" \
+  "$ROOT/target/debug/dsh-desktop"
+do
+  if [ -x "$candidate" ]; then
+    exec "$candidate" "$@"
+  fi
+done
+exec cargo run --manifest-path "$ROOT/Cargo.toml" -p dsh-desktop -- "$@"