feat: add official Codex skill and harden task execution

Install and update artemis-use with task diagnostics and plugin guidance. Preserve cross-device file receipts, review canonical reports, guard stage transitions and reduce small history summaries. Improve exploration lifecycle, request recovery and browser live connections.

tests/unit/agents/test_flash_cross_target_observation.py

@@ -22,6 +22,9 @@ def cross_target(monkeypatch):
         bindings={PC: ("pc", None)},
         tool_names={PC},
         previous_image=lambda *_: b"pc-before",
+        # This fixture only returns a PC observation; stage-write guards have
+        # dedicated coverage in test_flash_progress.py.
+        is_read_only=lambda _: True,
     )
     monkeypatch.setattr("artemis.plugins.runtime.get_plugin_runtime", lambda _: plugins)
     runner = flash.FlashRunner.__new__(flash.FlashRunner)