fish: add fisher+tide config to chezmoi dotfiles

[nvonbulow]

Summary

• add Fish plugin declarations at home/dot_config/fish/fish_plugins for Fisher and Tide v6
• add home/dot_config/fish/conf.d/00-tide-vars.fish to declaratively set tide_* universal prompt variables
• add home/dot_config/fish/conf.d/10-fisher-bootstrap.fish to bootstrap the fisher function on first interactive Fish run
• avoid Tide first-install wizard during startup by not auto-installing plugins in bootstrap; users run fisher update once to install from fish_plugins
• intentionally exclude runtime/internal variables (_tide*, _fisher*, __fish_initialized) and avoid committing fish_variables

Validation

• fish -n home/dot_config/fish/conf.d/00-tide-vars.fish
• fish -n home/dot_config/fish/conf.d/10-fisher-bootstrap.fish
• isolated startup test with temp HOME/XDG_CONFIG_HOME confirms:
  • no Tide configure wizard during startup
  • Fisher function is bootstrapped
  • Tide universal vars are set
• chezmoi diff ~/.config/fish/conf.d/00-tide-vars.fish ~/.config/fish/conf.d/10-fisher-bootstrap.fish ~/.config/fish/fish_plugins

Safety

• scoped changes only to Fish config under home/dot_config/fish
• no secrets/runtime state files committed (no fish_variables, no auth/session files)