Switch to overlay-only patching; remove sitecustomize.py

Patches to b2.py and restic.py are applied directly in the overlayfs at
PREINIT boot time. The sitecustomize.py import hook was belt-and-suspenders
that succeeded or failed alongside the file patch every time, providing no
genuine fallback.

- Delete patch/sitecustomize.py entirely
- apply.sh: remove sitecustomize install step; flatten if/elif/else structure;
  restore self-contained URL-fix logic in the restic.py BLOCK; rename overlay
  tag 'sc' -> 'mw'
- recover.sh: unmount overlays to restore original files immediately, no
  reboot required; kill-switch file prevents re-application on next boot
- uninstall.sh: remove sitecustomize.py removal section; update overlay tag
- install.sh: update preflight to check patch/apply.sh, not sitecustomize.py
- README: remove sitecustomize references throughout; update recovery docs

install.sh

@@ -20,7 +20,7 @@ set -euo pipefail
 PATCH_DIR="$(cd "$(dirname "$0")" && pwd)"
 _HOOK_COMMENT='TrueCloud provider patch (S3/B2)'
 
-if [ ! -f "$PATCH_DIR/patch/sitecustomize.py" ]; then
+if [ ! -f "$PATCH_DIR/patch/apply.sh" ]; then
     echo "ERROR: patch files not found at $PATCH_DIR/patch/" >&2
     echo "Run install.sh from a clone of the repository on a persistent pool:" >&2
     echo "  git clone https://github.com/sudolulo/truenas-truecloud-patch \\" >&2