feat: add sponge via fisher; gitignore fisher-managed files

meaningful-ooo/sponge is now the one true Fisher-managed plugin,
giving the bootstrap a concrete purpose. Its files (and functions/fisher.fish,
completions/fisher.fish) are gitignored so Fisher owns them exclusively —
no committed/managed conflict possible.

Updates docs, index, requirements, and README to distinguish
Fisher-managed plugins from bundled customized ones.

README.md

@@ -22,7 +22,7 @@ This config layers on top of the CachyOS base Fish configuration and adds:
 
 - **Catppuccin Mocha** theming throughout (prompt, FZF, syntax highlighting)
 - **Starship** prompt with VI key bindings
-- **Fisher** plugin manager bootstrapped automatically; plugin functionality (FZF bindings, Catppuccin theme, done notifications, autopair, puffer-fish) bundled directly with the config
+- **Fisher** plugin manager bootstrapped automatically; manages `sponge` (failed-command history filter); FZF bindings, Catppuccin theme, done, autopair, and puffer-fish are bundled directly with the config as customized versions
 - **Smart CLI wrappers** that prefer modern tools (`eza`, `bat`, `btop`, `dust`, `prettyping`) with graceful fallbacks
 - **Auto Python venv** activation on directory change (direnv-aware)
 - **Kitty terminal** deep integration for splits, tabs, and SSH