feat: Add Python plugins from AutoMetabuilder + restructure workflow folder

Restructure workflow/ for multi-language plugin support:
- Rename src/ to core/ (engine code: DAG executor, registry, types)
- Create executor/{cpp,python,ts}/ for language-specific runtimes
- Consolidate plugins to plugins/{ts,python}/ by language then category

Add 80+ Python plugins from AutoMetabuilder in 14 categories:
- control: bot control, switch logic, state management
- convert: type conversions (json, boolean, dict, list, number, string)
- core: AI requests, context management, tool calls
- dict: dictionary operations (get, set, keys, values, merge)
- list: list operations (concat, find, sort, slice, filter)
- logic: boolean logic (and, or, xor, equals, comparisons)
- math: arithmetic operations (add, subtract, multiply, power, etc.)
- string: string manipulation (concat, split, replace, format)
- notifications: Slack, Discord integrations
- test: assertion helpers and test suite runner
- tools: file operations, git, docker, testing utilities
- utils: filtering, mapping, reducing, condition branching
- var: variable store operations (get, set, delete, exists)
- web: Flask server, environment variables, JSON handling

Add language executor runtimes:
- TypeScript: direct import execution (default, fast startup)
- Python: child process with JSON stdin/stdout communication
- C++: placeholder for native FFI bindings (Phase 3)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

workflow/plugins/python/web/web_start_server.py

@@ -0,0 +1,26 @@
+"""Workflow plugin: start Flask server."""
+
+
+def run(runtime, inputs):
+    """Start the Flask web server.
+
+    Inputs:
+        host: Host address (default: 0.0.0.0)
+        port: Port number (default: 8000)
+        debug: Enable debug mode (default: False)
+
+    Returns:
+        dict: Success indicator (note: this blocks until server stops)
+    """
+    app = runtime.context.get("flask_app")
+    if not app:
+        return {"error": "Flask app not found in context. Run web.create_flask_app first."}
+
+    host = inputs.get("host", "0.0.0.0")
+    port = inputs.get("port", 8000)
+    debug = inputs.get("debug", False)
+
+    # This will block until the server is stopped
+    app.run(host=host, port=port, debug=debug)
+
+    return {"result": "Server stopped"}