fix: harden installer symlink logic and interactive sourcing

- install-claude-profiles: distinguish broken symlink vs real dir vs free
  path via -L/-e, repoint existing links with ln -sfn, warn instead of
  silently skipping when target is a non-symlink dir.
- profiles.sh: only set -euo pipefail when sourced non-interactively so
  sourcing in an interactive shell doesn't clobber session settings.

Co-Authored-By: Claude <noreply@anthropic.com>

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