chore(models): upgrade the opus alias to claude-opus-4-8

MODEL_MAP["opus"] moves off claude-opus-4-6 to the newest Opus tier at the
same price; pricing already matched via the claude-opus-4 fragment, and a
new test_opus_is_priced guard keeps the alias priced. Fixtures, panel
mocks, and docs follow.

tests/unit/runtime/test_spawn_manifest.py

@@ -87,7 +87,7 @@ class TestBuildForRole:
                 role=role,
                 team="board",
                 workspace_path=Path("/tmp/x"),
-                agent_model="claude-opus-4-6",
+                agent_model="claude-opus-4-8",
             )
         )
         assert "read_messages" in m.do_tools