fix(memory): discover user-installed memory providers from $HERMES_HOME/plugins/ (#10529)

Memory provider discovery (discover_memory_providers, load_memory_provider)
only scanned the bundled plugins/memory/ directory. User-installed providers
at $HERMES_HOME/plugins/<name>/ were invisible, forcing users to symlink
into the repo source tree — which broke on hermes update and created a
dual-registration path causing duplicate tool names (400 errors on strict
providers like Xiaomi MiMo).

Changes:
- Add _get_user_plugins_dir(), _is_memory_provider_dir(), _iter_provider_dirs(),
  and find_provider_dir() helpers to plugins/memory/__init__.py
- discover_memory_providers() now scans both bundled and user dirs
- load_memory_provider() uses find_provider_dir() (bundled-first)
- discover_plugin_cli_commands() uses find_provider_dir()
- _install_dependencies() in memory_setup.py uses find_provider_dir()
- User plugins use _hermes_user_memory namespace to avoid sys.modules collisions
- Non-memory user plugins filtered via source text heuristic
- Bundled providers always take precedence on name collisions

Fixes #4956, #9099. Supersedes #4987, #9123, #9130, #9132, #9982.

hermes_cli/memory_setup.py

@@ -58,9 +58,11 @@ def _prompt(label: str, default: str | None = None, secret: bool = False) -> str
 def _install_dependencies(provider_name: str) -> None:
     """Install pip dependencies declared in plugin.yaml."""
     import subprocess
-    from pathlib import Path as _Path
+    from plugins.memory import find_provider_dir
 
-    plugin_dir = _Path(__file__).parent.parent / "plugins" / "memory" / provider_name
+    plugin_dir = find_provider_dir(provider_name)
+    if not plugin_dir:
+        return
     yaml_path = plugin_dir / "plugin.yaml"
     if not yaml_path.exists():
         return