feat: introduce MCP server, cross-platform runtime, and Showcase UI reorganization

- Showcase UI Reorganization:
  * Move web UI dashboard from root directory into apps/showcase_ui/ for cleaner project structure
  * Add 'artemis ui' CLI command (artemis/interfaces/cli/commands/ui.py) and launcher scripts for the interactive showcase console
- Standalone MCP Server & Notifiers (mcp_server/):
  * Implement dedicated MCP server package with mobile automation tools (device_state, inspect_trace, task_manager, task_runner)
  * Add multi-channel notifiers (AgentAPI, Desktop notifications, Webhooks, File output, and Composite routing)
  * Introduce background task runner and trace store for autonomous mobile execution monitoring
- Cross-Platform Runtime & Toolchain Support:
  * Add artemis/platform/ with OS-specific implementations for Linux, macOS (Darwin), and Windows
  * Add artemis/runtime/supervisor.py for resilient process and agent runtime supervision
  * Add artemis/toolchain/ with dependency resolution and SDK descriptors
  * Add cross-platform launcher scripts (start.sh, start.bat) and installer scripts (scripts/install_deps.*)
- Core Enhancements & Diagnostics:
  * Enhance coordinate normalization, OCR API fallback, and perception pipeline
  * Add credential validation (credentials_validator.py) and system diagnostics
  * Update bilingual documentation (README.md, README_CN.md) and architecture diagrams
- Test Suite Expansion:
  * Add integration tests for cloud gateway, remote ADB, and Artemis login
  * Add unit tests covering core diagnostics, platform detection, toolchain, credentials validator, optional OCR, and MCP tools

artemis/config/core/engine.py

@@ -0,0 +1,118 @@
+# 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.
+
+"""Multi-source Cascading Configuration Engine."""
+
+import json
+from pathlib import Path
+import re
+from typing import Any
+
+from dotenv import load_dotenv
+
+from artemis.config.core.interpolator import interpolate_config_value
+from artemis.platform import platform
+from artemis.utils.logger import get_logger
+
+logger = get_logger(__name__)
+
+
+def _strip_json_comments(text: str) -> str:
+    """Strip // and /* */ comments from JSONC text while preserving string literals."""
+    pattern = r'//.*?$|/\*.*?\*/|\'(?:\\.|[^\\\'])*\'|"(?:\\.|[^\\"])*"'
+
+    def replacer(match: re.Match) -> str:
+        s = match.group(0)
+        if s.startswith("/"):
+            return ""
+        return s
+
+    return re.sub(pattern, replacer, text, flags=re.DOTALL | re.MULTILINE)
+
+
+def _deep_merge(base: dict[str, Any], override: dict[str, Any]) -> dict[str, Any]:
+    """Recursively deep merge two dictionaries."""
+    merged = base.copy()
+    for key, value in override.items():
+        if key in merged and isinstance(merged[key], dict) and isinstance(value, dict):
+            merged[key] = _deep_merge(merged[key], value)
+        else:
+            merged[key] = value
+    return merged
+
+
+class CascadingConfigEngine:
+    """Manages 5-tier configuration hierarchy with automatic cascade and interpolation."""
+
+    def __init__(self, workspace_root: Path | None = None):
+        self.workspace_root = workspace_root or Path.cwd()
+        self._load_environment_files()
+
+    def _load_environment_files(self) -> None:
+        """Load .env files from global config dir and workspace root."""
+        # 1. Global user .env
+        global_env = platform.paths.config_dir / ".env"
+        if global_env.exists():
+            load_dotenv(dotenv_path=global_env, override=False, encoding="utf-8")
+
+        # 2. Legacy ~/.artemis/.env fallback
+        legacy_env = platform.paths.data_dir / ".env"
+        if legacy_env.exists() and legacy_env != global_env:
+            load_dotenv(dotenv_path=legacy_env, override=False, encoding="utf-8")
+
+        # 3. Workspace .env
+        ws_env = self.workspace_root / ".env"
+        if ws_env.exists():
+            load_dotenv(dotenv_path=ws_env, override=True, encoding="utf-8")
+
+    def load_raw_config(
+        self,
+        cli_overrides: dict[str, Any] | None = None,
+    ) -> dict[str, Any]:
+        """Load and merge all config layers into a unified dictionary."""
+        config: dict[str, Any] = {}
+
+        # Tier 1: Global user configuration ($CONFIG_DIR/artemis.jsonc)
+        global_config_path = platform.paths.config_dir / "artemis.jsonc"
+        if global_config_path.exists():
+            try:
+                raw_text = global_config_path.read_text(encoding="utf-8")
+                cleaned = _strip_json_comments(raw_text)
+                config = _deep_merge(config, json.loads(cleaned))
+            except Exception as e:
+                logger.warning(f"Failed to load global config {global_config_path}: {e}")
+
+        # Tier 2: Workspace project configuration (config/artemis.jsonc)
+        ws_config_path = self.workspace_root / "config" / "artemis.jsonc"
+        if not ws_config_path.exists():
+            ws_config_path = self.workspace_root / "artemis.jsonc"
+
+        if ws_config_path.exists():
+            try:
+                raw_text = ws_config_path.read_text(encoding="utf-8")
+                cleaned = _strip_json_comments(raw_text)
+                config = _deep_merge(config, json.loads(cleaned))
+            except Exception as e:
+                logger.warning(f"Failed to load workspace config {ws_config_path}: {e}")
+
+        # Tier 3: CLI overrides
+        if cli_overrides:
+            config = _deep_merge(config, cli_overrides)
+
+        # Apply variable interpolation across merged dictionary
+        interpolated = interpolate_config_value(config, workspace_root=self.workspace_root)
+        return interpolated
+
+
+config_engine = CascadingConfigEngine()