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.yml

@@ -89,6 +89,9 @@ services:
       # Optional: persist VS Code server and extensions across container recreations
       # - devbox-vscode:/home/developer/.vscode-server
 
+      # Persist neovim plugin/Mason data (avoids re-downloading on every recreate)
+      - devbox-nvim-data:/home/developer/.local/share/nvim
+
       # Optional: AWS credentials/SSO config (not read-only — SSO writes token cache)
       # - ~/.aws:/home/developer/.aws
 
@@ -97,6 +100,7 @@ volumes:
   devbox-state:
   devbox-shell-history:
   devbox-zoxide:
+  devbox-nvim-data:
   devbox-uv:
   # devbox-rustup:
   # devbox-cargo: