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.

docs/shell/scripts.mdx

@@ -68,7 +68,7 @@ atuin-pick
 
 Opens atuin's interactive search (`atuin search -i --filter-mode directory`), limited to history from the current directory. It then `exec`s an interactive `fish`. If you picked a command, the new fish runs `echo '❯ <cmd>'; <cmd>` via `-C` (`--init-command`), so aliases resolve, a `cd` persists, and the prompt appears below the output. Esc starts a plain fish.
 
-It is written for launch contexts where atuin's shell init never ran. It resolves `atuin` from `PATH` or `~/.atuin/bin/atuin`, and seeds `ATUIN_SESSION` from `atuin uuid` if unset.
+It is written for launch contexts where atuin's shell init never ran. It runs `atuin` from `PATH` and seeds `ATUIN_SESSION` from `atuin uuid` if unset.
 
 **Launched by:** <kbd>ctrl+shift+p</kbd> in kitty. `kitty.conf` opens it in a horizontal split (`launch_hs atuin-pick`). Under the kitty-tmux profile, `tmux.conf` runs it with `tmux split-window -v -l 25%` in the current pane's directory. See [kitty](/terminal/kitty) and [atuin](/shell/atuin).