refactor: remove dead image media attachment code

- Remove generated_image_paths_from_messages() and _extract_text_payload() from artifacts.py (no runtime callers)
- Remove session_attachments.py entirely (merge_turn_media_into_last_assistant and stage_media_paths_for_session_replay had no runtime callers)
- Remove test_session_media_persist.py and the orphaned test in test_artifacts.py

tests/agent/test_session_media_persist.py

@@ -1,34 +0,0 @@
-"""Tests for staging attachment paths into the media bucket for session replay."""
-
-from pathlib import Path
-
-from nanobot.config.loader import set_config_path
-from nanobot.config.paths import get_media_dir
-from nanobot.utils.session_attachments import stage_media_paths_for_session_replay
-
-
-def test_persist_media_stages_workspace_file(tmp_path: Path) -> None:
-    set_config_path(tmp_path / "config.json")
-    outside = tmp_path / "workspace" / "report.md"
-    outside.parent.mkdir(parents=True)
-    outside.write_text("body", encoding="utf-8")
-
-    out = stage_media_paths_for_session_replay([str(outside)])
-
-    assert len(out) == 1
-    staged = Path(out[0])
-    assert staged.is_file()
-    assert staged.read_text(encoding="utf-8") == "body"
-    assert staged.resolve().is_relative_to(get_media_dir().resolve())
-
-
-def test_persist_media_keeps_files_already_under_media_root(tmp_path: Path) -> None:
-    set_config_path(tmp_path / "config.json")
-    media = get_media_dir("websocket")
-    media.mkdir(parents=True, exist_ok=True)
-    inside = media / "keep-me.txt"
-    inside.write_text("x", encoding="utf-8")
-
-    out = stage_media_paths_for_session_replay([str(inside.resolve())])
-
-    assert out == [str(inside.resolve())]