fix(plugins): an undeclared OS is not told to install the app

A portable plugin with no app declaration for this OS maps to missing_app,
so describe() told the user to install an app that cannot fix the OS
restriction. It now says the app is not supported on this operating system.

tests/hermes_cli/test_plugins_cmd.py

@@ -25,7 +25,7 @@ from hermes_cli.plugins_cmd import (
 from hermes_cli.plugins_cmd_install import _refuse_unavailable_portable_plugin
 
 
-def _write_portable_app_plugin(root: Path, app: Path) -> None:
+def _write_portable_app_plugin(root: Path, app: Path, os_key: str | None = None) -> None:
     from hermes_cli.agent_plugins import MCP_SCHEMA_V1, PLUGIN_SCHEMA_V1
     from hermes_platform.host.facts import os_family
 
@@ -33,7 +33,7 @@ def _write_portable_app_plugin(root: Path, app: Path) -> None:
         "$schema": PLUGIN_SCHEMA_V1,
         "name": "example-plugin",
         "extensions": {"com.nousresearch.hermes": {"servers": {"worker": {
-            "app": {os_family(): {"presence": "executable", "location": str(app)}},
+            "app": {os_key or os_family(): {"presence": "executable", "location": str(app)}},
             "requires": {"app": True},
         }}}},
     }), encoding="utf-8")
@@ -54,6 +54,16 @@ def test_portable_install_gate_accepts_present_app_and_refuses_missing(tmp_path:
         _refuse_unavailable_portable_plugin("example-plugin", tmp_path)
 
 
+def test_portable_install_gate_on_undeclared_os_does_not_say_install(tmp_path: Path) -> None:
+    from hermes_platform.host.facts import os_family
+
+    other = next(o for o in ("linux", "darwin", "win32") if o != os_family())
+    _write_portable_app_plugin(tmp_path, tmp_path / "example-app", os_key=other)
+    with pytest.raises(PluginOperationError, match="Example App is not supported on this operating system") as exc:
+        _refuse_unavailable_portable_plugin("example-plugin", tmp_path, "Example App")
+    assert "Install" not in str(exc.value)
+
+
 # ── _sanitize_plugin_name ─────────────────────────────────────────────────
 
 

tools/mcp_liveness.py

@@ -180,7 +180,10 @@ def describe(decl: declaration.Declaration, available: Availability, liveness_st
     app_name = display_name or decl.name
     gpu = declaration.gpu_label(decl.required_gpu)
     action, _retry = _action(liveness_state, app_name, gpu)
-    if liveness_state == "missing_app":
+    if liveness_state == "missing_app" and available.state == "unsupported_os":
+        reason = f"{app_name} is not supported on this operating system."
+        action = "Use a supported operating system."
+    elif liveness_state == "missing_app":
         reason = f"{app_name} is not installed."
         if available.min_version:
             action = f"Install {app_name} {available.min_version} or newer, then try again."