fix(bindings): trigger fzf inline picker with a lookbehind @ instead of a @@ chord

Binding the raw @@ chord made a lone @ an ambiguous prefix, so fish
(with fish_sequence_key_delay_ms unset) held every typed @ indefinitely
until a disambiguating keystroke arrived, breaking things like
`ssh user@host`. Binding the single @ key instead and checking whether
the current token is already a bare @ removes the ambiguity entirely:
plain @ always self-inserts instantly, and a second consecutive @
triggers the picker in place, with no forced delimiter before you can
keep typing. Vi's normal/visual modes are left unbound, matching their
existing (no-op) @ behavior.

docs/manual/03-key-bindings.md

@@ -42,9 +42,11 @@ are active in Insert, Normal, and Visual modes unless noted.
                     pressing Enter a second time for certain fast-path
                     commands (speedtest-fast, etc.).
 
-    @@              FZF inline picker. Type @@ anywhere on the command
-                    line to open an fzf picker and insert a selection
-                    at the cursor position.
+    @@              FZF inline picker. Type @ twice anywhere on the
+                    command line to open an fzf picker and replace the
+                    @@ with the selection. The @@ must be typed as its
+                    own token: "test @@" triggers it, but "test@@" does
+                    not.
 
     Ctrl+Right      Accept autosuggestion one word/directory segment
                     at a time. (Restores Fish 3.x behavior by binding