test(plugins): cover _discover_all_plugins recursion + cross-link loader

Add a TestDiscoverAllPlugins class covering the six cases the recursive
scan needs to handle:

- flat plugin uses its manifest ``name:`` as the key
- category-namespaced plugin keys off ``<category>/<dirname>`` even when
  the manifest ``name:`` is bare (regression test for the original bug —
  ``plugins/observability/langfuse/`` with ``name: langfuse`` must
  surface as ``observability/langfuse``, not ``langfuse``)
- user-installed plugin overrides bundled on key collision
- depth cap: anything below ``<root>/<category>/<plugin>/`` is ignored
- bundled ``memory/`` and ``context_engine/`` are skipped (they have
  their own loaders), but user plugins under those category names are
  still scanned

Also add an in-source comment next to the key derivation pointing at the
loader's matching line (``PluginManager._parse_manifest`` in
plugins.py:1027-1028), so future renames of one site flag the other.

Both items raised in Copilot review on #27161.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hermes_cli/plugins_cmd.py

@@ -761,6 +761,11 @@ def _discover_all_plugins() -> list:
                         description = manifest.get("description", "")
                     except Exception:
                         pass
+                # Path-derived key, intentionally ignoring the manifest
+                # ``name:`` field for category-namespaced plugins — mirrors
+                # ``PluginManager._parse_manifest`` in plugins.py:1027-1028
+                # so renaming a directory (without touching plugin.yaml) shifts
+                # the registry key in both places consistently.
                 key = f"{prefix}/{d.name}" if prefix else manifest_name
                 src_label = source
                 if source == "user" and (d / ".git").exists():