Upgrade default Claude Opus model to 4.8

Bump the default Opus route/model from claude-opus-4-7 to claude-opus-4-8
across provider presets (claude, claudeDesktop, hermes, openclaw, opencode,
universal), i18n locales (zh/en/ja/zh-TW), pricing seed data, and the
user-manual docs.

- Add claude-opus-4-8 pricing row ($5/$25/$0.50/$6.25); keep the 4-7 row
  for historical usage stats (seeded via INSERT OR IGNORE).
- Claude Desktop proxy: accept bidirectional opus 4-7 <-> 4-8 route alias
  during rollout so previously saved routes keep resolving.
- thinking_optimizer: route opus-4-8 through adaptive thinking and normalize
  dotted model ids (also fixes dotted 4-6/4-7 falling back to legacy).
- usage_stats: normalize Bedrock/Vertex/aggregator opus-4-8 ids to base
  pricing.

Also merge role:"system" messages into the Gemini systemInstruction in the
Anthropic->Gemini transform.

tests/config/therouterProviderPresets.test.ts

@@ -24,7 +24,7 @@ describe("TheRouter provider presets", () => {
     expect(env.ANTHROPIC_DEFAULT_SONNET_MODEL).toBe(
       "anthropic/claude-sonnet-4.6",
     );
-    expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe("anthropic/claude-opus-4.7");
+    expect(env.ANTHROPIC_DEFAULT_OPUS_MODEL).toBe("anthropic/claude-opus-4.8");
   });
 
   it("uses the OpenAI-compatible v1 endpoint for Codex", () => {