fix: harden installer symlink logic and interactive sourcing (#1)

lib/profiles.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
-set -euo pipefail
+# Only enforce strict mode when sourced non-interactively, so sourcing this
+# file in an interactive shell doesn't clobber the user's session settings.
+[[ $- == *i* ]] || set -euo pipefail
 
 repo_root() {
   local script_dir