DAG action executor

execute_action_dag() runs actions in dependency order. Each node is {"id": str, "action": [name, ...], "depends_on": [id, ...]}. Independent branches fan out across a thread pool; when a node fails, its transitive dependents are marked skipped (fail_fast=True, the default) or still run (fail_fast=False):

from automation_file import execute_action_dag

results = execute_action_dag([
    {"id": "fetch",  "action": ["FA_download_file",
                                ["https://example.com/src.tar.gz", "src.tar.gz"]]},
    {"id": "verify", "action": ["FA_verify_checksum",
                                ["src.tar.gz", "3b0c44298fc1..."]],
                     "depends_on": ["fetch"]},
    {"id": "unpack", "action": ["FA_unzip_file", ["src.tar.gz", "src"]],
                     "depends_on": ["verify"]},
    {"id": "report", "action": ["FA_fast_find", ["src", "*.py"]],
                     "depends_on": ["unpack"]},
])

Cycles, unknown dependencies, self-dependencies, and duplicate ids raise DagException before any node runs. The JSON-action form is FA_execute_action_dag.