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.

.gitignore

@@ -62,6 +62,15 @@
 opinionated_catalog.md
 docs/superpowers
 
+#   ──────────────── Fisher-Managed Plugin Files ───────────────
+# Fisher writes these into the repo directory on install/update.
+# They are owned by Fisher — do not commit them.
+functions/fisher.fish
+completions/fisher.fish
+conf.d/sponge.fish
+functions/_sponge*.fish
+functions/sponge_*.fish
+
 #   ─────────────────── AI Sessions and Rules ──────────────────
 # Matches CLAUDE.md, .claud*, etc.
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