fix: add Adapter alias + qualify bare runtime imports

Two compounding bugs from the post-#87 extraction caught by today's
template sweep (matches the same fixes already shipped in claude-code
and gemini-cli):

1. adapter.py never aliased LangGraphAdapter to the contract name
   `Adapter`, which `molecule_runtime.adapters.get_adapter()` reads
   via `getattr(mod, "Adapter")`. Without it, every workspace
   startup fails preflight with "no \`Adapter\` class is exported".

2. Two bare imports of runtime modules
   (`from agent`, `from a2a_executor`) never got qualified to
   `from molecule_runtime.X import Y`. They worked when runtime was
   bundled into workspace/ but explode in the standalone template
   repo with ModuleNotFoundError.

Same migration debt class fixed for hermes (a2a-sdk migration),
claude-code (5 imports), gemini-cli (4 imports + alias) earlier today.
Both gates now in molecule-ci PR #8 will catch this class of bug
at PR time going forward.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

adapter.py

@@ -43,8 +43,11 @@ class LangGraphAdapter(BaseAdapter):
         self.system_prompt = result.system_prompt
 
     async def create_executor(self, config: AdapterConfig) -> AgentExecutor:
-        from agent import create_agent
-        from a2a_executor import LangGraphA2AExecutor
+        from molecule_runtime.agent import create_agent
+        from molecule_runtime.a2a_executor import LangGraphA2AExecutor
 
         agent = create_agent(config.model, self.all_tools, self.system_prompt)
         return LangGraphA2AExecutor(agent, heartbeat=config.heartbeat, model=config.model)
+
+
+Adapter = LangGraphAdapter