feat(mcp): enhance multi-platform notification resilience and reactive wakeups

- AgentApiNotifier: Sort candidate process sessions by creation time descending to prioritize the latest active IDE session over stale PIDs. Implement automatic retry across candidate address/token pairs and persist working credentials to shared environment files (~/.gemini/jetski/.jetski_env and ~/.artemis/.artemis_env).
- DesktopNotifier: Enable native OS desktop toast notifications by default on macOS, Linux, and Windows unless explicitly disabled or running in CI. Ensure MCP installer sets ARTEMIS_DESKTOP_NOTIFY=true in generated IDE configurations.
- ScriptNotifier: Add universal command hook adapter (ARTEMIS_NOTIFY_CMD / MCP_NOTIFY_COMMAND) for custom IDE and CLI integration.
- Fault Isolation: Wrap CompositeNotifier and global notify entrypoint in strict try-except blocks to guarantee zero-crash execution even if any individual notifier fails.
- Tests & Docs: Add unit tests for session recovery, fault tolerance, script hook execution, and document multi-platform notification architecture in README.

mcp_server/notifiers/script.py

@@ -0,0 +1,90 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Custom Script / Command Hook Notifier for universal IDE and editor integration."""
+
+import logging
+import os
+import subprocess
+from typing import Any
+
+from mcp_server.notifiers.base import BaseNotifier
+
+logger = logging.getLogger("mcp_server.notifiers.script")
+
+
+class ScriptNotifier(BaseNotifier):
+    """Notifier that executes a user-defined command or script when an event occurs.
+
+    This adapter enables universal integration with any custom IDE, editor (Neovim/Emacs),
+    or automation platform by allowing users to define ARTEMIS_NOTIFY_CMD or MCP_NOTIFY_COMMAND.
+    Placeholders like {title}, {message}, {conversation_id}, {event_type}, and {trace_id}
+    are automatically replaced before execution.
+    """
+
+    ENV_VARS = [
+        "ARTEMIS_NOTIFY_CMD",
+        "MCP_NOTIFY_COMMAND",
+    ]
+
+    @property
+    def name(self) -> str:
+        return "script"
+
+    def _get_command_template(self) -> str | None:
+        for var in self.ENV_VARS:
+            val = os.getenv(var)
+            if val and val.strip():
+                return val.strip()
+        return None
+
+    def is_available(self) -> bool:
+        return self._get_command_template() is not None
+
+    def notify(
+        self,
+        conversation_id: str,
+        message: str,
+        title: str | None = None,
+        event_type: str = "completed",
+        payload: dict[str, Any] | None = None,
+    ) -> bool:
+        cmd_template = self._get_command_template()
+        if not cmd_template:
+            return False
+
+        trace_id = (payload or {}).get("trace_id", "")
+        formatted_title = title or f"Artemis Task {event_type.capitalize()}"
+
+        # Replace template placeholders safely
+        try:
+            cmd = (
+                cmd_template.replace("{title}", str(formatted_title))
+                .replace("{message}", str(message))
+                .replace("{conversation_id}", str(conversation_id))
+                .replace("{event_type}", str(event_type))
+                .replace("{trace_id}", str(trace_id))
+            )
+            subprocess.run(
+                cmd,
+                shell=True,
+                stdout=subprocess.DEVNULL,
+                stderr=subprocess.DEVNULL,
+                timeout=10,
+            )
+            logger.info(f"Custom script notification command executed: {cmd[:60]}...")
+            return True
+        except Exception as e:
+            logger.warning(f"Failed to execute custom script notification: {e}")
+            return False