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/split-1G.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -e
+
+for f in "$@"; do
+    split --verbose --suffix-length=5 --bytes=1G --numeric-suffixes "$f" "$f."
+done