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_config/atuin/atuin-receipt.json

@@ -1 +0,0 @@
-{"binaries":["atuin"],"binary_aliases":{},"cdylibs":[],"cstaticlibs":[],"install_layout":"flat","install_prefix":"/home/prad/.atuin/bin","modify_path":true,"provider":{"source":"cargo-dist","version":"0.31.0"},"source":{"app_name":"atuin","name":"atuin","owner":"atuinsh","release_type":"github"},"version":"18.19.0"}

dot_config/fish/conf.d/atuin.env.fish

@@ -1,2 +0,0 @@
-
-source "$HOME/.atuin/bin/env.fish"

dot_config/fish/config.fish

@@ -35,7 +35,8 @@ test -s ~/.config/envman/load.fish; and source ~/.config/envman/load.fish
 
 if status is-interactive
     atuin init fish | source
-    atuin ai init fish | source
+    # atuin 18.21 dropped `atuin ai init`; the `?` binding it used to emit is
+    # copied below as _atuin_ai_question_mark.
 end
 
 # cmux