feat(fish): add remote deploy helper

tests/deploy-to.bash

@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+function_path=$1
+workdir=$(mktemp -d)
+trap 'rm -rf "$workdir"' EXIT
+
+export HOME="$workdir/home"
+mkdir -p "$HOME"
+
+git config --global user.email test@example.invalid
+git config --global user.name 'Deploy Test'
+
+origin="$workdir/origin.git"
+repo="$workdir/repo"
+ssh_log="$workdir/ssh.log"
+mock_bin="$workdir/bin"
+
+git init --bare "$origin" >/dev/null
+git clone "$origin" "$repo" >/dev/null 2>&1
+
+cd "$repo"
+git switch -c impl/test >/dev/null
+echo test >README.md
+git add README.md
+git commit -m 'test fixture' >/dev/null
+git push -u origin impl/test >/dev/null 2>&1
+
+mkdir -p "$mock_bin"
+bash_path=${BASH_PATH:-$(command -v bash)}
+cat >"$mock_bin/ssh" <<SSH
+#!$bash_path
+set -euo pipefail
+printf '%s\n' "\$1" >"\$SSH_LOG"
+printf '%s\n' "\$2" >>"\$SSH_LOG"
+SSH
+chmod +x "$mock_bin/ssh"
+
+export SSH_LOG="$ssh_log"
+export PATH="$mock_bin:$PATH"
+
+fish_definition="function deploy-to; source '$function_path' \$argv; end"
+default_command="$fish_definition; deploy-to pikachu"
+explicit_command="$fish_definition; deploy-to pikachu impl/test"
+
+fish -c "$default_command"
+
+expected_command='cd ~/wrk/dotfiles && git fetch origin && git switch -- impl/test && git reset --hard origin/impl/test && sudo nixos-rebuild switch --flake .#pikachu --print-build-logs'
+
+mapfile -t ssh_call <"$ssh_log"
+
+test "${ssh_call[0]}" = pikachu
+test "${ssh_call[1]}" = "$expected_command"
+
+rm "$ssh_log"
+fish -c "$explicit_command"
+mapfile -t ssh_call <"$ssh_log"
+
+test "${ssh_call[0]}" = pikachu
+test "${ssh_call[1]}" = "$expected_command"
+
+rm "$ssh_log"
+echo dirty >>README.md
+
+if fish -c "$default_command" 2>"$workdir/dirty.err"
+then
+    echo 'deploy-to unexpectedly allowed a dirty worktree' >&2
+    exit 1
+fi
+
+grep -F 'dirty local worktree' "$workdir/dirty.err" >/dev/null
+
+git checkout -- README.md
+
+git commit --allow-empty -m 'unpushed fixture' >/dev/null
+
+if fish -c "$default_command" 2>"$workdir/unpushed.err"
+then
+    echo 'deploy-to unexpectedly allowed a branch behind origin' >&2
+    exit 1
+fi
+
+grep -F 'local HEAD differs from origin/impl/test' "$workdir/unpushed.err" >/dev/null