ask before replacing a config the user already has, and show the difference

Keeping theirs silently was safe but unhelpful: they never learn a newer version
exists, and the only hint was a cp command printed in a warning. Now it asks.

  [1] keep yours — nothing changes
  [2] use ours — yours is kept as <file>.before-machine-setup
  [3] show me the difference first

The diff is labelled "yours" and "ours" rather than by path, so - is what you
would lose and + is what you would gain, and it goes through the pager because a
config diff is routinely longer than a screen. Choosing to replace always keeps
the old file beside the new one; nothing is destroyed.

An unattended run — ASSUME_YES, or no terminal on stdin — keeps theirs and says
so. "Yes to everything" cannot sensibly mean "overwrite configuration nobody was
present to defend", so this is the one prompt ASSUME_YES answers conservatively
rather than affirmatively.

Applies to every file that goes through install_config, which is .tmux.conf and
starship.toml today and is where any other dotfile should go.

On the starship question: there is one file now, scripts/setup/starship.toml, and
the inline copy is gone. Owner and members get the same prompt, which is what
os-user-shell.ts always claimed.

Verified through a pty, since the -t 0 guard correctly makes the interactive path
untestable over a pipe: the diff renders, replacing writes the backup, and the
live file ends up byte-identical to ours.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/machine-setup/lib/files.sh

@@ -19,17 +19,19 @@
 [[ -n "${MACHINE_SETUP_FILES_LOADED:-}" ]] && return 0
 MACHINE_SETUP_FILES_LOADED=1
 
-# Put a config file in place, unless the user has their own.
+# Put a config file in place, asking before it replaces one the user has.
 #
 # Three outcomes, and the caller can tell them apart by the return code:
 #
-#   0  installed — there was nothing there
+#   0  installed — there was nothing there, or the user chose to replace
 #   1  identical — already exactly this, nothing done
-#   2  kept      — theirs differs, left alone
+#   2  kept      — the user chose to keep theirs
 #
-# Converging when there is nothing to lose and keeping what the user wrote when
-# there is. The third case prints how to take ours, so the choice stays with
-# them.
+# When the file exists and differs, this ASKS rather than deciding. Silently
+# keeping theirs is safe but unhelpful — they never learn that a newer version
+# exists — and silently replacing it is how a setup script eats somebody's
+#configuration. So: keep, replace, or show the difference first, and a replaced file is
+# always kept beside the new one.
 #
 # NOTE for callers: 1 and 2 are outcomes, not failures — but they are still
 # non-zero, so calling this as a plain command under `set -e` ends the script
@@ -37,19 +39,57 @@ MACHINE_SETUP_FILES_LOADED=1
 #
 #   install_config "$src" "$dest" "$user" && rc=0 || rc=$?
 install_config() {
-  local src="$1" dest="$2" owner="$3"
+  local src="$1" dest="$2" owner="$3" answer
 
-  if [[ -f "$dest" ]]; then
-    if cmp -s "$src" "$dest"; then
-      return 1
-    fi
-    warn "kept your ${dest} — it differs from the one shipped here"
-    echo "         to take ours instead: cp ${src} ${dest}"
+  if [[ ! -f "$dest" ]]; then
+    install -D -m 0644 -o "$owner" -g "$owner" "$src" "$dest"
+    return 0
+  fi
+
+  cmp -s "$src" "$dest" && return 1
+
+  echo ""
+  warn "${dest} already exists here, and differs from the one this script ships."
+
+  # Never replace a file the user has without being told to. An unattended run
+  # answers "keep", because the alternative is destroying configuration nobody
+  # was present to defend.
+  if [[ "${ASSUME_YES:-}" == "1" ]] || [[ ! -t 0 ]]; then
+    echo "     keeping yours (nothing was asked, so nothing is replaced)"
     return 2
   fi
 
-  install -D -m 0644 -o "$owner" -g "$owner" "$src" "$dest"
-  return 0
+  while true; do
+    echo "     [1] keep yours — nothing changes"
+    echo "     [2] use ours — yours is kept as ${dest}.before-machine-setup"
+    echo "     [3] show me the difference first"
+    echo ""
+    if ! read -rp "  Which one? (1/2/3) [1]: " answer; then
+      echo ""
+      echo "     keeping yours"
+      return 2
+    fi
+    case "${answer:-1}" in
+      1)
+        echo "     keeping yours"
+        return 2
+        ;;
+      2)
+        cp -a "$dest" "${dest}.before-machine-setup"
+        install -D -m 0644 -o "$owner" -g "$owner" "$src" "$dest"
+        ok "replaced — yours is at ${dest}.before-machine-setup"
+        return 0
+        ;;
+      3)
+        echo ""
+        # yours on the left, ours on the right: - is what you would lose,
+        # + is what you would gain.
+        diff -u --label "yours: ${dest}" --label "ours: ${src}" "$dest" "$src" | page
+        echo ""
+        ;;
+      *) warn "Pick 1, 2 or 3." ;;
+    esac
+  done
 }
 
 # Append a block to a file exactly once.