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/manifest.py

@@ -20,6 +20,7 @@ Bottle schema (frontmatter):
   egress: { routes: [ <egress-route>, ... ] }
     # route keys: host, matches, auth, role, dlp
   supervise:    <bool>                          # optional (default true)
+  nested_containers: <bool>                     # optional (default false)
 
 Agent schema (frontmatter):
   bottle:        <bottle-name>          # required