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/fish.mdx

@@ -43,7 +43,6 @@ fish sources `conf.d/*.fish` in name order and then `config.fish`. The numbered
 | `30-ssh-agent.fish` | One ssh-agent shared across shells |
 | `40-tools.fish` | mise, television, starship, zoxide, entire, hishtory, unsloth |
 | `50-terminal-title.fish` | tmux/screen window name from `$PWD` |
-| `atuin.env.fish` | Sources `~/.atuin/bin/env.fish` (puts atuin's install dir on `PATH`) |
 | `inir-env.fish` | `INIR_VENV` and `ILLOGICAL_IMPULSE_VIRTUAL_ENV` → `~/.local/state/quickshell/.venv` |
 | `inir-path.fish` | Prepends `~/.local/bin` to `PATH` if missing |
 | `nvm.fish` | nvm.fish plugin events and default-version activation |
@@ -169,10 +168,10 @@ The header comment says the file should stay empty, but it holds the following:
 2. **Bottom-pinned prompt.** In interactive shells with `FISH_BOTTOM_PROMPT=1`, `tput cup $LINES 0` moves the cursor to the last row. The comment calls `1` the default, but `10-env.fish` sets `0`, so the prompt stays at the top unless you export `FISH_BOTTOM_PROMPT=1`.
 3. `fish_add_path ~/.grok/bin` (grok installer block).
 4. Sources `~/.config/envman/load.fish` if it is non-empty.
-5. **atuin.** Interactive shells run `atuin init fish | source` and `atuin ai init fish | source`. See [atuin](/shell/atuin).
+5. **atuin.** Interactive shells run `atuin init fish | source`. See [atuin](/shell/atuin).
 6. `fish_add_path ~/.cmux/bin` and `~/.cache/.bun/bin`.
 7. Sources the first readable `~/.config/herdr/plugins/github/herdr-automatic-rename-*/shell/hook.fish`. This is herdr's automatic pane-rename hook ([herdr](/terminal/herdr)).
-8. A copy of the `_atuin_ai_question_mark` function that `atuin ai init fish` emits, followed by `bind "?" _atuin_ai_question_mark`.
+8. A copy of the `_atuin_ai_question_mark` function that `atuin ai init fish` used to emit (atuin 18.21 removed that command), followed by `bind "?" _atuin_ai_question_mark`.
 
 ## Key bindings