fix: no auto-activation + unified hermes plugins UI with provider categories

- Remove auto-activation: when context.engine is 'compressor' (default),
  plugin-registered engines are NOT used. Users must explicitly set
  context.engine to a plugin name to activate it.

- Add curses_radiolist() to curses_ui.py: single-select radio picker
  with keyboard nav + text fallback, matching curses_checklist pattern.

- Rewrite cmd_toggle() as composite plugins UI:
  Top section: general plugins with checkboxes (existing behavior)
  Bottom section: provider plugin categories (Memory Provider, Context Engine)
  with current selection shown inline. ENTER/SPACE on a category opens
  a radiolist sub-screen for single-select configuration.

- Add provider discovery helpers: _discover_memory_providers(),
  _discover_context_engines(), config read/save for memory.provider
  and context.engine.

- Add tests: radiolist non-TTY fallback, provider config save/load,
  discovery error handling, auto-activation removal verification.

run_agent.py

@@ -1304,13 +1304,7 @@ class AIAgent:
                     "Context engine '%s' not found — falling back to built-in compressor",
                     _engine_name,
                 )
-        else:
-            # Even with default config, check if a plugin registered one
-            try:
-                from hermes_cli.plugins import get_plugin_context_engine
-                _selected_engine = get_plugin_context_engine()
-            except Exception:
-                pass
+        # else: config says "compressor" — use built-in, don't auto-activate plugins
 
         if _selected_engine is not None:
             self.context_compressor = _selected_engine