fix(smoke): reseed mcpplibs cleanly, excluding the repo's read-only .git packs

smoke-full-linux failed in 'smoke tests' with hundreds of:
  cp: cannot create regular file '.../mcpplibs/./.git/objects/pack/pack-*.pack': Permission denied

Root cause: the smoke jobs run several scripts under one shared MCPP_HOME. By the
time smoke_imgui_module.sh runs, an earlier smoke has already populated
$MCPP_HOME/registry/data/mcpplibs/.git with read-only git pack objects (git
makes packs mode 0444). smoke_imgui_module.sh then did, with no guard:
  cp -a "$ROOT/." "$default_index/"
which copies the repo's OWN .git (same-named, read-only packs) over the existing
read-only packs — and cp cannot overwrite a read-only target -> Permission
denied -> the whole job fails (exit 1). Pre-existing since #51; not a mcpp/xlings
bug (mcpp shells no such cp; xlings uses std::filesystem::copy).

Fix: remove the destination first (rm -rf tolerates read-only files — it needs
write on the parent dir, not the file), and seed from the working tree EXCLUDING
the repo's .git. The package index only needs pkgs/; the repo's read-only pack
objects are both irrelevant and the sole cause of the conflict.

Reproduced locally: old path -> 821 'Permission denied'; new path -> 0, pkgs/
present.

Author: Sunrisepeak

tests/smoke_imgui_module.sh

@@ -51,8 +51,15 @@ if [[ -f "$USER_MCPP/config.toml" ]]; then
 fi
 
 default_index="$MCPP_HOME/registry/data/mcpplibs"
+# Reseed cleanly. The smoke jobs share one MCPP_HOME across several scripts, so
+# an earlier smoke may already have populated mcpplibs/.git with read-only pack
+# objects; `cp -a "$ROOT/."` over those then fails with "Permission denied".
+# Remove the destination first, and skip the repo's own .git — its pack objects
+# are read-only and irrelevant to the package index (which only needs pkgs/),
+# and are the sole reason the copy ever conflicts.
+rm -rf "$default_index"
 mkdir -p "$default_index"
-cp -a "$ROOT/." "$default_index/"
+( cd "$ROOT" && find . -mindepth 1 -maxdepth 1 ! -name .git -exec cp -a {} "$default_index/" \; )
 rm -f "$default_index/.xlings-index-cache.json"
 printf 'ok\n' > "$default_index/.mcpp-index-updated"