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

tests/test_archetypes.py

@@ -266,7 +266,7 @@ def test_ini_subsection_direct_match_unaffected():
 # ---------------------------------------------------------------------------
 
 def test_build_deckies_archetype_sets_services():
-    from decnet.cli import _build_deckies
+    from decnet.fleet import build_deckies as _build_deckies
     from decnet.archetypes import get_archetype
     arch = get_archetype("mail-server")
     result = _build_deckies(
@@ -283,7 +283,7 @@ def test_build_deckies_archetype_sets_services():
 
 
 def test_build_deckies_archetype_preferred_distros():
-    from decnet.cli import _build_deckies
+    from decnet.fleet import build_deckies as _build_deckies
     from decnet.archetypes import get_archetype
     arch = get_archetype("iot-device")  # preferred_distros=["alpine"]
     result = _build_deckies(
@@ -298,7 +298,7 @@ def test_build_deckies_archetype_preferred_distros():
 
 
 def test_build_deckies_explicit_services_override_archetype():
-    from decnet.cli import _build_deckies
+    from decnet.fleet import build_deckies as _build_deckies
     from decnet.archetypes import get_archetype
     arch = get_archetype("linux-server")
     result = _build_deckies(