fix(macos-container): make backend the macos default

docs/prds/prd-new-macos-container-backend.md

@@ -7,13 +7,12 @@
 
 ## Summary
 
-Add an experimental `macos-container` backend that integrates Apple's
-`container` CLI as a host runtime on macOS. The first shipped slice
-registers the backend and implements reusable host primitives
-(`build`, `exec`, `cp`, image inspection, cleanup, active
-enumeration). Follow-up slices make launch runnable with the proven
-two-network sidecar topology and add real-runtime coverage, without
-weakening bot-bottle's sidecar egress model.
+Add a `macos-container` backend that integrates Apple's `container`
+CLI as a host runtime on macOS. The shipped slices register the
+backend, implement reusable host primitives (`build`, `exec`, `cp`,
+image inspection, cleanup, active enumeration), make launch runnable
+with the proven two-network sidecar topology, and add real-runtime
+coverage without weakening bot-bottle's sidecar egress model.
 
 ## Problem
 
@@ -44,6 +43,8 @@ path around the egress sidecar.
 - `--backend=macos-container` and
   `BOT_BOTTLE_BACKEND=macos-container` are accepted by the existing
   backend selector.
+- Compatible macOS hosts default to `macos-container` when
+  `BOT_BOTTLE_BACKEND` and `--backend` are both unset.
 - Backend availability is true only on macOS hosts with `container` on
   `PATH`.
 - The backend has tested wrappers for Apple Container image build,
@@ -62,7 +63,7 @@ path around the egress sidecar.
 ## Non-goals
 
 - Do not remove or deprecate the Docker backend.
-- Do not change the default backend from `smolmachines`.
+- Do not remove or deprecate the smolmachines backend.
 - Do not run sidecar daemons as host processes.
 - Do not launch a degraded backend where the agent can bypass the
   egress sidecar through direct network access.