Persist neovim plugin/Mason data across container recreations

Mason LSP installs and Lazy plugin cache live at ~/.local/share/nvim,
which was in the container's writable layer. Every --force-recreate
triggered a full re-download of all plugins and LSP servers on next
nvim launch — slow and wasteful.

Add devbox-nvim-data named volume in docker-compose.yml and
docker-compose.shared.yml, add to entrypoint ownership-fix loop,
update persistence tables in README.md and DOCKER_HUB.md.

docker-compose.shared.yml

@@ -55,6 +55,9 @@ services:
       # Persist zoxide directory history ('z <fragment>' to jump)
       - devbox-zoxide:/home/developer/.local/share/zoxide
 
+      # Persist neovim plugin/Mason data (avoids re-downloading on every recreate)
+      - devbox-nvim-data:/home/developer/.local/share/nvim
+
       # Persist uv data (Python installs)
       - devbox-uv:/home/developer/.local/share/uv
 
@@ -65,4 +68,5 @@ volumes:
   devbox-data:
   devbox-shell-history:
   devbox-zoxide:
+  devbox-nvim-data:
   devbox-uv: