fix: address Copilot PR review comments

- Move save_init_options() before preset install so skills propagation
  works during 'specify init --preset --ai-skills'
- Clean up downloaded ZIP after successful preset install during init
- Validate --from URL scheme (require HTTPS, HTTP only for localhost)
- Expose unregister_commands() on extensions.py CommandRegistrar wrapper
  instead of reaching into private _registrar field
- Use _get_merged_packs() for search() and get_pack_info() so all
  active catalogs are searched, not just the highest-priority one
- Fix fetch_catalog() cache to verify cached URL matches current URL
- Fix PresetResolver: script resolution uses .sh extension, consistent
  file extensions throughout resolve(), and resolve_with_source()
  delegates to resolve() to honor template_type parameter
- Fix bash common.sh: fall through to directory scan when python3
  returns empty preset list
- Fix PowerShell Resolve-Template: filter out dot-folders and sort
  extensions deterministically

scripts/bash/common.sh

@@ -190,9 +190,16 @@ except Exception:
                     local candidate="$presets_dir/$preset_id/templates/${template_name}.md"
                     [ -f "$candidate" ] && echo "$candidate" && return 0
                 done <<< "$sorted_presets"
+            else
+                # python3 returned empty list — fall through to directory scan
+                for preset in "$presets_dir"/*/; do
+                    [ -d "$preset" ] || continue
+                    local candidate="$preset/templates/${template_name}.md"
+                    [ -f "$candidate" ] && echo "$candidate" && return 0
+                done
             fi
         else
-            # Fallback: alphabetical directory order
+            # Fallback: alphabetical directory order (no python3 available)
             for preset in "$presets_dir"/*/; do
                 [ -d "$preset" ] || continue
                 local candidate="$preset/templates/${template_name}.md"