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.

docs/fish-config.index

@@ -166,6 +166,8 @@ universal=## Fish Universal Variables
 # ── Section 8: Fisher Plugins ─────────────────────────────────
 plugins=# 8. FISHER PLUGINS
 fisher=# 8. FISHER PLUGINS
+fisher-managed=## Fisher-Managed Plugins
+sponge=## Fisher-Managed Plugins
 bundled-plugins=## Bundled Plugin Functionality
 fish-plugins-manifest=## fish_plugins Manifest