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/string/__init__.py

@@ -0,0 +1 @@
+"""String manipulation plugins."""

workflow/plugins/python/string/string_concat.py

@@ -0,0 +1,10 @@
+"""Workflow plugin: concatenate strings."""
+
+
+def run(_runtime, inputs):
+    """Concatenate multiple strings."""
+    strings = inputs.get("strings", [])
+    separator = inputs.get("separator", "")
+
+    str_list = [str(s) for s in strings]
+    return {"result": separator.join(str_list)}

workflow/plugins/python/string/string_format.py

@@ -0,0 +1,13 @@
+"""Workflow plugin: format string with variables."""
+
+
+def run(_runtime, inputs):
+    """Format string with variables."""
+    template = inputs.get("template", "")
+    variables = inputs.get("variables", {})
+
+    try:
+        result = template.format(**variables)
+        return {"result": result}
+    except (KeyError, ValueError) as e:
+        return {"result": template, "error": str(e)}

workflow/plugins/python/string/string_length.py

@@ -0,0 +1,7 @@
+"""Workflow plugin: get string length."""
+
+
+def run(_runtime, inputs):
+    """Get length of a string."""
+    text = inputs.get("text", "")
+    return {"result": len(text)}

workflow/plugins/python/string/string_lower.py

@@ -0,0 +1,7 @@
+"""Workflow plugin: convert string to lowercase."""
+
+
+def run(_runtime, inputs):
+    """Convert string to lowercase."""
+    text = inputs.get("text", "")
+    return {"result": text.lower()}

workflow/plugins/python/string/string_replace.py

@@ -0,0 +1,12 @@
+"""Workflow plugin: replace in string."""
+
+
+def run(_runtime, inputs):
+    """Replace occurrences in string."""
+    text = inputs.get("text", "")
+    old = inputs.get("old", "")
+    new = inputs.get("new", "")
+    count = inputs.get("count", -1)
+
+    result = text.replace(old, new, count)
+    return {"result": result}

workflow/plugins/python/string/string_split.py

@@ -0,0 +1,15 @@
+"""Workflow plugin: split string."""
+
+
+def run(_runtime, inputs):
+    """Split string by separator."""
+    text = inputs.get("text", "")
+    separator = inputs.get("separator", " ")
+    max_splits = inputs.get("max_splits")
+
+    if max_splits is not None:
+        result = text.split(separator, max_splits)
+    else:
+        result = text.split(separator)
+
+    return {"result": result}

workflow/plugins/python/string/string_trim.py

@@ -0,0 +1,16 @@
+"""Workflow plugin: trim whitespace from string."""
+
+
+def run(_runtime, inputs):
+    """Trim whitespace from string."""
+    text = inputs.get("text", "")
+    mode = inputs.get("mode", "both")
+
+    if mode == "start":
+        result = text.lstrip()
+    elif mode == "end":
+        result = text.rstrip()
+    else:
+        result = text.strip()
+
+    return {"result": result}

workflow/plugins/python/string/string_upper.py

@@ -0,0 +1,7 @@
+"""Workflow plugin: convert string to uppercase."""
+
+
+def run(_runtime, inputs):
+    """Convert string to uppercase."""
+    text = inputs.get("text", "")
+    return {"result": text.upper()}