fix(atuin): use the packaged binary only

~/.atuin/bin held an old standalone atuin (18.19.0) that the shells
prepended to PATH, shadowing pacman's /usr/bin/atuin (18.21.0). The
daemon runs the packaged binary and migrated the history DB, so every
shell command through the old one failed with "migration 20260818000000
was previously applied but is missing in the resolved migrations".

Drop conf.d/atuin.env.fish, the `. ~/.atuin/bin/env` lines in .bashrc
and .zshrc (atuin init is now guarded on `command -v atuin`), the
cargo-dist receipt, and atuin-pick's ~/.atuin/bin fallback. Also drop
`atuin ai init fish`, which 18.21 removed; config.fish already carries
the `?` binding it emitted.

dot_local/bin/executable_atuin-pick

@@ -9,12 +9,11 @@ output in the split, and the prompt drops down below it.
 """
 import os
 import shlex
-import shutil
 import subprocess
 
-# kitty's launch env has neither ~/.atuin/bin on PATH nor ATUIN_SESSION
-# (atuin's shell init never ran), so resolve and seed both here.
-atuin = shutil.which("atuin") or os.path.expanduser("~/.atuin/bin/atuin")
+# kitty's launch env has no ATUIN_SESSION (atuin's shell init never ran), so
+# seed it here. atuin itself comes from the package manager, on PATH.
+atuin = "atuin"
 if "ATUIN_SESSION" not in os.environ:
     os.environ["ATUIN_SESSION"] = subprocess.run(
         [atuin, "uuid"], stdout=subprocess.PIPE, text=True