tb-scripts: add .sh extensions and add ,yt-dlp wrapper

Rename all shell script files to use .sh extensions for consistency
and better editor support (pushover-send, split-1G, xz, zipdir).
Update default.nix references to match the new filenames.

Add a new ,yt-dlp script that runs yt-dlp from nixpkgs-unstable
via `nix run`, so it's always up to date without needing to be
in the system packages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkgs/tb-scripts/xz.sh

@@ -0,0 +1,13 @@
+#!/bin/sh -x
+
+PROCS=$(grep -c ^processor /proc/cpuinfo)
+
+if [ $# -eq 0 ]; then
+    (for f in *; do
+         echo "$f";
+     done) | xargs -P"$PROCS" -d'\n' -n1 xz -9ve
+else
+    (for f in "$@"; do
+         echo "$f";
+     done) | xargs -P"$PROCS" -d'\n' -n1 xz -9ve
+fi