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_zshrc

@@ -80,9 +80,8 @@ for _zsh_hl in \
 done
 unset _zsh_hl
 
-. "$HOME/.atuin/bin/env"
-
-eval "$(atuin init zsh)"
+# atuin comes from the package manager (/usr/bin); see ~/.bashrc.
+command -v atuin >/dev/null 2>&1 && eval "$(atuin init zsh)"
 
 # >>> termium >>>
 case ":$PATH:" in *:'/home/prad/.local/bin':*) ;; *) export PATH='/home/prad/.local/bin':"$PATH" ;; esac