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/interpolator.py

@@ -0,0 +1,78 @@
+# 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.
+
+"""Dynamic configuration variable interpolation engine (${env:VAR}, ${path:NAME})."""
+
+import os
+from pathlib import Path
+import re
+from typing import Any
+
+from artemis.platform import platform
+
+_VAR_PATTERN = re.compile(r"\$\{([^}]+)\}")
+
+
+def _resolve_variable(expr: str, workspace_root: Path | None = None) -> str:
+    """Resolve a single ${namespace:key:-default} expression."""
+    expr = expr.strip()
+    namespace, _, rest = expr.partition(":")
+    if not rest:
+        # Defaults to environment variable lookup
+        return os.getenv(namespace, f"${{{expr}}}")
+
+    key, _, default = rest.partition(":-")
+
+    if namespace == "env":
+        return os.getenv(key, default)
+    elif namespace == "path":
+        paths = platform.paths
+        if key == "config_dir":
+            return str(paths.config_dir)
+        elif key == "data_dir":
+            return str(paths.data_dir)
+        elif key == "cache_dir":
+            return str(paths.cache_dir)
+        elif key == "logs_dir":
+            return str(paths.logs_dir)
+        elif key == "temp_dir":
+            return str(paths.temp_dir())
+        elif key == "workspace_root" or key == "root_dir":
+            return str(workspace_root or Path.cwd())
+        return default or f"${{{expr}}}"
+    elif namespace == "platform":
+        if key == "os_type":
+            return platform.os_type.value
+        elif key == "path_sep":
+            return platform.process.path_separator
+        return default or f"${{{expr}}}"
+
+    return os.getenv(expr, default or f"${{{expr}}}")
+
+
+def interpolate_config_value(val: Any, workspace_root: Path | None = None) -> Any:
+    """Recursively interpolate strings, lists, and dicts containing ${...} variables."""
+    if isinstance(val, str):
+        if "${" not in val:
+            return val
+
+        def replacer(match: re.Match) -> str:
+            return _resolve_variable(match.group(1), workspace_root)
+
+        return _VAR_PATTERN.sub(replacer, val)
+    elif isinstance(val, dict):
+        return {k: interpolate_config_value(v, workspace_root) for k, v in val.items()}
+    elif isinstance(val, list):
+        return [interpolate_config_value(item, workspace_root) for item in val]
+    return val