fix(manifest): OR nested_containers across composed bottles

merge_bottles_runtime sees resolved bottles, where a bottle that never
mentioned the key is indistinguishable from one that set it false. With
"later replaces" semantics, `--bottle with-containers --bottle claude-dev`
silently dropped the capability — the second bottle's default clobbered
it. OR instead: composing bottles adds capabilities, it does not remove
them. The file-based extends path still sees raw keys, so an explicit
`nested_containers: false` in a child continues to turn it back off.

Also surface the flag in the launch summary, where its absence was the
only reason the clobber went unnoticed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/cli/start.py

@@ -524,6 +524,8 @@ def _manifest_to_yaml(manifest: Manifest) -> str:
                 lines.append(f"          scheme: {r.AuthScheme}")
 
     lines.append(f"  supervise: {'true' if bottle.supervise else 'false'}")
+    if bottle.nested_containers:
+        lines.append("  nested_containers: true")
 
     return "\n".join(lines)