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/billing/test_pricing.py

@@ -565,6 +565,7 @@ class TestProviderAwareness:
         for name in (
             "claude-opus-4-6",
             "claude-opus-4-8",
+            "claude-opus-5",
             "claude-fable-5",
             "opus",
             "sonnet",