fix: harden vim template startup checks

[dmoliveira]

Summary

• guard optional Vim plugin-manager and statusline hooks so shipped templates start cleanly without local plugin installs
• fall back to a built-in colorscheme when solarized is unavailable
• add CI smoke coverage for both Vim templates and install vim explicitly for that check

Validation

• vim -Nu run_commands/my_vimrc -n -es +qa!
• vim -Nu run_commands/.my_vimrc -n -es +qa!
• lua -e "assert(loadfile('run_commands/my_nvim_init.lua'))"