fix(plugins): copy history entries out, lock clear_state

Review follow-ups on the transition history.

get_state_history() copied only the outer list, so a caller holding a
returned transition could rewrite the manager's record of what happened
-- which contradicted the defensive-copy guarantee in its own docstring.
Copy each entry too. Every value in a transition is immutable, so a
shallow copy per entry is enough. test_get_state_history_entries_are_copies
pins it; without the change it fails with 'tampered' == 'enabled'.

clear_state() mutated five shared dicts without holding _lock, while
every other mutator takes it. A concurrent set_state() could interleave
and leave a plugin with history but no state. Drop the five as one unit.

This does not close the wider unload-vs-worker race, which lives in
PluginManager.unload_plugin() and predates this change: an update worker
still in flight can call set_state() after clear_state() returns and
recreate the entry. Serialising that needs the per-plugin lock held
across worker join in unload_plugin(), which is a separate change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/plugin_system/plugin_state.py

@@ -167,11 +167,16 @@ class PluginStateManager:
             plugin_id: Plugin identifier
 
         Returns:
-            List of recent state transitions, oldest first. A copy, so callers
-            cannot mutate the manager's own history.
+            List of recent state transitions, oldest first. Both the list and
+            the transition dicts are copies, so callers cannot mutate the
+            manager's own history. The values inside a transition are all
+            immutable, so a shallow copy per entry is enough.
         """
         with self._lock:
-            return list(self._state_history.get(plugin_id, ()))
+            return [
+                dict(transition)
+                for transition in self._state_history.get(plugin_id, ())
+            ]
     
     def set_error_info(self, plugin_id: str, error_info: Dict[str, Any]) -> None:
         """
@@ -290,11 +295,17 @@ class PluginStateManager:
         return info
     
     def clear_state(self, plugin_id: str) -> None:
-        """Clear all state information for a plugin."""
-        self._states.pop(plugin_id, None)
-        self._state_history.pop(plugin_id, None)
-        self._state_transition_counts.pop(plugin_id, None)
-        self._error_info.pop(plugin_id, None)
-        self._last_update.pop(plugin_id, None)
-        self._last_display.pop(plugin_id, None)
+        """Clear all state information for a plugin.
+
+        Held under ``_lock`` so the five dicts are dropped as one unit: every
+        other mutator takes the lock, and without it a concurrent set_state()
+        could interleave and leave a plugin with history but no state.
+        """
+        with self._lock:
+            self._states.pop(plugin_id, None)
+            self._state_history.pop(plugin_id, None)
+            self._state_transition_counts.pop(plugin_id, None)
+            self._error_info.pop(plugin_id, None)
+            self._last_update.pop(plugin_id, None)
+            self._last_display.pop(plugin_id, None)