Gentoo bootstrap: fix ambiguous package names + early-return on os_family mismatch

Three issues caught during tadbit onboarding:

1. run_once_00-install-bootstrap-tools.sh.tmpl (gentoo branch):
   'gnupg' is ambiguous (app-crypt/gnupg vs app-vim/gnupg). Use full
   category/package names: app-crypt/gnupg, app-crypt/age, etc.
   The previous 'for p in age curl ...' loop fed short names to
   emerge which printed '!!! The short ebuild name gnupg is
   ambiguous' and exited 1.

2. run_once_20-install-user-packages-gentoo.sh.tmpl:
   sys-devel/base-devel doesn't exist on Gentoo (it's an Arch/Fedora
   concept; Gentoo's toolchain is the @system set which is always
   installed). Removed base-devel from the package list.

3. Universal scripts (run_once_20 + run_once_40) were running on
   gentoo and hitting either 'die unsupported os_family' (universal
   20) or 'WARNING sway packages not configured for gentoo' (universal
   40). Added early-return: 'if os_family == gentoo, exit 0' at the
   top of each universal script so the gentoo-specific scripts
   handle the box. (Same pattern the chaotic-aur script already had.)

4. run_once_10-add-gentoo-overlays.sh.tmpl:
   The cached ~/.cache/eselect-repo/repositories.xml was corrupt on
   tadbit (lxml.etree.XMLSyntaxError on every 'eselect repository
   list' call). Added a sanity check: parse the XML with
   xml.etree.ElementTree, delete if invalid, re-fetch.

After these fixes, the bootstrap on tadbit is expected to run cleanly
with --keep-going (the four failures above all become no-ops).

run_once_20-install-user-packages-gentoo.sh.tmpl

@@ -56,6 +56,9 @@ fi
 # we'll install via oh-my-zsh custom plugins (gentoo's ebuilds can lag).
 
 # Step 2: Define the package set
+# Note: on Gentoo there's no `base-devel` meta-package. The toolchain
+# (gcc, binutils, glibc, make, patch, etc.) is part of the @system set
+# which is always installed. We only need to list user-space packages.
 GENTOO_PKGS=(
     app-shells/zsh
     app-shells/zsh-completions
@@ -63,7 +66,6 @@ GENTOO_PKGS=(
     app-admin/tmux
     app-editors/neovim
     dev-vcs/git
-    sys-devel/base-devel
     app-text/bat
     sys-process/btop
     sys-process/htop
@@ -74,8 +76,8 @@ GENTOO_PKGS=(
     sys-apps/ripgrep
     app-shells/zoxide
     app-shells/starship
-    # lazygit is in GURU (app-vim/lazygit? actually app-misc/lazygit in guru)
-    # topgrade is in GURU
+    # lazygit is in GURU (app-misc/lazygit)
+    # topgrade is in GURU (app-misc/topgrade)
     # media-video/yt-dlp is in main
     app-text/jq
     app-arch/unzip