feat(shell): add fzf inline picker bound to @@

Adds __fzf_inline_picker function and @@ keybinding (Emacs + all Vi
modes). Opens an fzf session and injects the selected item at the
cursor position in the current command line. Includes SYNOPSIS/
DESCRIPTION doc block per project conventions. Updates README with
new binding in the User Bindings table.

functions/__fzf_inline_picker.fish

@@ -0,0 +1,29 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __fzf_inline_picker
+#
+# DESCRIPTION
+#   Opens an interactive fzf session and injects the selected item directly
+#   into the command line at the cursor position. Bound to @@ by default.
+#   Requires fzf to be installed. Repaints the prompt after insertion.
+#
+# RETURNS
+#   0  A selection was made and inserted into the command line
+#   0  No selection was made (fzf cancelled); command line is unchanged
+#
+# EXAMPLE
+#   # Press @@ at the prompt — fzf opens, pick an item, it appears at cursor
+#
+function __fzf_inline_picker
+    # Open fzf and capture selection
+    set -l selection (fzf)
+
+    if test -n "$selection"
+        # Injects text instantly at the cursor
+        commandline -i (string escape -- $selection)
+    end
+    # Refresh the line display
+    commandline -f repaint
+end