fix(plugins+nous): auto-coerce memory plugins; actionable Nous 401 diagnostic (#14005)

* fix(plugins): auto-coerce user-installed memory plugins to kind=exclusive

User-installed memory provider plugins at $HERMES_HOME/plugins/<name>/
were being dispatched to the general PluginManager, which has no
register_memory_provider method on PluginContext. Every startup logged:

  Failed to load plugin 'mempalace': 'PluginContext' object has no
  attribute 'register_memory_provider'

Bundled memory providers were already skipped via skip_names={memory,
context_engine} in discover_and_load, but user-installed ones weren't.

Fix: _parse_manifest now scans the plugin's __init__.py source for
'register_memory_provider' or 'MemoryProvider' (same heuristic as
plugins/memory/__init__.py:_is_memory_provider_dir) and auto-coerces
kind to 'exclusive' when the manifest didn't declare one explicitly.
This routes the plugin to plugins/memory discovery instead of the
general loader.

The escape hatch: if a manifest explicitly declares kind: standalone,
the heuristic doesn't override it.

Reported by Uncle HODL on Discord.

* fix(nous): actionable CLI message when Nous 401 refresh fails

Mirrors the Anthropic 401 diagnostic pattern. When Nous returns 401
and the credential refresh (_try_refresh_nous_client_credentials)
also fails, the user used to see only the raw APIError. Now prints:

  🔐 Nous 401 — Portal authentication failed.
     Response: <truncated body>
     Most likely: Portal OAuth expired, account out of credits, or
                  agent key revoked.
     Troubleshooting:
       • Re-authenticate: hermes login --provider nous
       • Check credits / billing: https://portal.nousresearch.com
       • Verify stored credentials: $HERMES_HOME/auth.json
       • Switch providers temporarily: /model <model> --provider openrouter

Addresses the common 'my hermes model hangs' pattern where the user's
Portal OAuth expired and the CLI gave no hint about the next step.

tests/hermes_cli/test_plugins.py

@@ -250,6 +250,73 @@ class TestPluginLoading:
 
         assert "hermes_plugins.ns_plugin" in sys.modules
 
+    def test_user_memory_plugin_auto_coerced_to_exclusive(self, tmp_path, monkeypatch):
+        """User-installed memory plugins must NOT be loaded by the general
+        PluginManager — they belong to plugins/memory discovery.
+
+        Regression test for the mempalace crash:
+            'PluginContext' object has no attribute 'register_memory_provider'
+
+        A plugin that calls ``ctx.register_memory_provider`` in its
+        ``__init__.py`` should be auto-detected and treated as
+        ``kind: exclusive`` so the general loader records the manifest but
+        does not import/register() it. The real activation happens through
+        ``plugins/memory/__init__.py`` via ``memory.provider`` config.
+        """
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        plugin_dir = plugins_dir / "mempalace"
+        plugin_dir.mkdir(parents=True)
+        # No explicit `kind:` — the heuristic should kick in.
+        (plugin_dir / "plugin.yaml").write_text(yaml.dump({"name": "mempalace"}))
+        (plugin_dir / "__init__.py").write_text(
+            "class MemPalaceProvider:\n"
+            "    pass\n"
+            "def register(ctx):\n"
+            "    ctx.register_memory_provider('mempalace', MemPalaceProvider)\n"
+        )
+        # Even if the user explicitly enables it in config, the loader
+        # should still treat it as exclusive and skip general loading.
+        hermes_home = tmp_path / "hermes_test"
+        (hermes_home / "config.yaml").write_text(
+            yaml.safe_dump({"plugins": {"enabled": ["mempalace"]}})
+        )
+        monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        assert "mempalace" in mgr._plugins
+        entry = mgr._plugins["mempalace"]
+        assert entry.manifest.kind == "exclusive", (
+            f"Expected auto-coerced kind='exclusive', got {entry.manifest.kind}"
+        )
+        # Not loaded by general manager (no register() call, no AttributeError).
+        assert not entry.enabled
+        assert entry.module is None
+        assert "exclusive" in (entry.error or "").lower()
+
+    def test_explicit_standalone_kind_not_coerced(self, tmp_path, monkeypatch):
+        """If a plugin explicitly declares ``kind: standalone`` in its
+        manifest, the memory-provider heuristic must NOT override it —
+        even if the source happens to mention ``MemoryProvider``.
+        """
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        plugin_dir = plugins_dir / "not_memory"
+        plugin_dir.mkdir(parents=True)
+        (plugin_dir / "plugin.yaml").write_text(
+            yaml.dump({"name": "not_memory", "kind": "standalone"})
+        )
+        (plugin_dir / "__init__.py").write_text(
+            "# This plugin inspects MemoryProvider docs but isn't one.\n"
+            "def register(ctx):\n    pass\n"
+        )
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "hermes_test"))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        assert mgr._plugins["not_memory"].manifest.kind == "standalone"
+
 
 # ── TestPluginHooks ────────────────────────────────────────────────────────