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
 

docs/fish-config.md

@@ -1417,12 +1417,21 @@ testing), run:
 
 Then open a new shell.
 
+## Fisher-Managed Plugins
+
+The following plugins are fully managed by Fisher. Their files are installed
+into the repo directory by Fisher and are listed in `.gitignore` — do not
+commit them. Fisher installs and updates them automatically.
+
+    jorgebucaran/fisher           Plugin manager itself
+    meaningful-ooo/sponge         Remove failed commands from history
+
 ## Bundled Plugin Functionality
 
-Plugin functionality is bundled directly with this config rather than managed
-through Fisher. The bundled versions include customizations for Fish 4.x
-compatibility and improved behavior that differ from their upstream releases.
-Installing them through Fisher would overwrite these customizations.
+The remaining plugin functionality is bundled directly with this config rather
+than managed through Fisher. The bundled versions include customizations for
+Fish 4.x compatibility and improved behavior that differ from their upstream
+releases. Installing them through Fisher would overwrite these customizations.
 
 Bundled components and their upstream origins:
 
@@ -1437,13 +1446,13 @@ versions. To update their behavior, edit the relevant bundled files directly.
 
 ## fish_plugins Manifest
 
-The `fish_plugins` file at the config root contains only the Fisher manager
-entry itself:
+The `fish_plugins` file at the config root:
 
     jorgebucaran/fisher           Plugin manager itself
+    meaningful-ooo/sponge         Remove failed commands from history
 
-To update Fisher, run `fisher update` or `fish-deps update` which calls it
-as its first step.
+To update all Fisher-managed plugins, run `fisher update` or `fish-deps
+update` which calls it as its first step.
 
 ---