feat: support multiple parents in bottle extends:

Allow extends: to accept a list of bottle names in addition to a plain
string. Parents are resolved independently and folded left-to-right
into a single combined parent before the child is merged on top, so
orthogonal concerns (base env, networking, agent provider) can live in
separate bottles without forcing a linear chain.

Merge rules for the parent fold: env dict-merge with later winning on
collision; git-gate.user per-field overlay; git-gate.repos union by
name with later winning per-field on same name; egress.routes
concatenated; all scalar fields (supervise, agent_provider, egress.log)
use last-wins. The existing child-wins-over-all-parents rule is
unchanged. Cycle detection, diamond deduplication, and missing/invalid
parent errors all work across multi-parent graphs.

Closes #268

bot_bottle/manifest_loader.py

@@ -87,5 +87,7 @@ def load_bottle_chain_from_dir(
         parent = fm.get("extends")
         if isinstance(parent, str):
             to_load.append(parent)
+        elif isinstance(parent, list):
+            to_load.extend(p for p in parent if isinstance(p, str))
 
     return resolve_bottles(raws)[bottle_name]