fix: qualify all bare imports of runtime modules

Five `from <runtime_module> import` statements in adapter.py +
claude_sdk_executor.py were never qualified when the template was
extracted to its own repo (#87). They worked when the runtime was
bundled into workspace/ where bare imports resolved against
sibling files; in the template repo they explode at startup with
ModuleNotFoundError as soon as Python reaches the import.

Caught by manual provision after pipeline-3 wire-real E2E. The
plugins import was the first one tripped because it sits in
adapter.setup() — earlier bare imports inside claude_sdk_executor.py
are deferred until the executor is constructed.

Pattern: any `from <X> import Y` where X is a workspace/ module ->
`from molecule_runtime.X import Y`. Fixes:
- adapter.py:97          plugins
- claude_sdk_executor.py executor_helpers, heartbeat, a2a_client, platform_auth

Same class of bug as the runtime's TOP_LEVEL_MODULES drift but
inverted — instead of forgetting to rewrite imports IN the wheel,
the template authors forgot to qualify imports IN the template
code (the build script's rewriter only runs on workspace/ -> wheel).

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

adapter.py

@@ -94,7 +94,7 @@ class ClaudeCodeAdapter(BaseAdapter):
         ``CLAUDE.md`` and ``/configs/skills/`` natively, and the default
         :class:`AgentskillsAdaptor` writes to both.
         """
-        from plugins import load_plugins
+        from molecule_runtime.plugins import load_plugins
         workspace_plugins_dir = os.path.join(config.config_path, "plugins")
         plugins = load_plugins(
             workspace_plugins_dir=workspace_plugins_dir,

claude_sdk_executor.py

@@ -41,7 +41,7 @@ from a2a.server.agent_execution import AgentExecutor, RequestContext
 from a2a.server.events import EventQueue
 from a2a.helpers import new_agent_text_message
 
-from executor_helpers import (
+from molecule_runtime.executor_helpers import (
     CONFIG_MOUNT,
     MEMORY_CONTENT_MAX_CHARS,
     WORKSPACE_MOUNT,
@@ -62,7 +62,7 @@ from executor_helpers import (
 )
 
 if TYPE_CHECKING:
-    from heartbeat import HeartbeatLoop
+    from molecule_runtime.heartbeat import HeartbeatLoop
 
 logger = logging.getLogger(__name__)
 
@@ -210,8 +210,8 @@ async def _report_tool_use(block: Any) -> None:
         # executor must still run when the workspace's network/auth
         # plumbing isn't fully set up (e.g. unit tests).
         import httpx
-        from a2a_client import PLATFORM_URL, WORKSPACE_ID
-        from platform_auth import auth_headers
+        from molecule_runtime.a2a_client import PLATFORM_URL, WORKSPACE_ID
+        from molecule_runtime.platform_auth import auth_headers
     except Exception:
         return
     try: