refactor: separate engine, collector, mutator, and fleet into independent subpackages

- decnet/engine/ — container lifecycle (deploy, teardown, status); _kill_api removed
- decnet/collector/ — Docker log streaming (moved from web/collector.py)
- decnet/mutator/ — mutation engine (no longer imports from cli or duplicates deployer code)
- decnet/fleet.py — shared decky-building logic extracted from cli.py

Cross-contamination eliminated:
- web router no longer imports from decnet.cli
- mutator no longer imports from decnet.cli
- cli no longer imports from decnet.web
- _kill_api() moved to cli (process management, not engine concern)
- _compose_with_retry duplicate removed from mutator

decnet/engine/__init__.py

@@ -0,0 +1,15 @@
+from decnet.engine.deployer import (
+    COMPOSE_FILE,
+    _compose_with_retry,
+    deploy,
+    status,
+    teardown,
+)
+
+__all__ = [
+    "COMPOSE_FILE",
+    "_compose_with_retry",
+    "deploy",
+    "status",
+    "teardown",
+]