fix: return None from image_created_at when timestamp is absent

FROM-scratch images (commit_container output) and registry images built
for reproducibility often omit the created field entirely. Both backends
were calling die() in that case, crashing the cached-image quickstart
before any container started.

image_created_at now returns datetime | None — None when the field is
absent or unparseable, die() only on real inspect failures (non-zero
exit, malformed JSON). stale_checks in both backends skips the staleness
check when None is returned.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/backend/docker/launch.py

@@ -231,7 +231,11 @@ def stale_checks(plan: DockerBottlePlan) -> None:
         return
     committed = read_committed_image(plan.slug)
     if committed and docker_mod.image_exists(committed):
-        check_stale(f"agent image {committed!r}", docker_mod.image_created_at(committed))
+        ts = docker_mod.image_created_at(committed)
+        if ts is not None:
+            check_stale(f"agent image {committed!r}", ts)
         return
     if docker_mod.image_exists(plan.image):
-        check_stale(f"agent image {plan.image!r}", docker_mod.image_created_at(plan.image))
+        ts = docker_mod.image_created_at(plan.image)
+        if ts is not None:
+            check_stale(f"agent image {plan.image!r}", ts)