chore(models): move the opus alias to claude-opus-5

Claude Opus 5 released today — same $5/$25 sticker, 1M context; Opus 4.8
moves to legacy. The pricing table gains a dedicated claude-opus-5
fragment (the claude-opus-4 substring doesn't cover it, so without the
row the fleet's Opus usage would silently cost-track as $0 — exactly
what test_opus_is_priced now guards).

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-4-8",
+                agent_model="claude-opus-5",
             )
         )
         assert "read_messages" in m.do_tools