fix(kiro): report 1M context window for claude-opus-4.8

Add 1M context capability overrides for claude-opus-4.8 and -thinking
variants, and use the resolved capability contextWindow (fallback 200k)
instead of the hardcoded 200k estimate in the Kiro executor.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/capabilities.test.js

@@ -0,0 +1,12 @@
+import { describe, expect, it } from "vitest";
+import { getCapabilitiesForModel } from "../../open-sse/providers/capabilities.js";
+
+describe("getCapabilitiesForModel", () => {
+  it("reports Kiro Claude Opus 4.8 as a 1M context model", () => {
+    expect(getCapabilitiesForModel("kiro", "claude-opus-4.8").contextWindow).toBe(1000000);
+    expect(getCapabilitiesForModel("kiro", "anthropic/claude-opus-4.8").contextWindow).toBe(1000000);
+    expect(getCapabilitiesForModel("kiro", "claude-opus-4-8").contextWindow).toBe(1000000);
+    expect(getCapabilitiesForModel("kiro", "claude-opus-4.8-thinking").contextWindow).toBe(1000000);
+    expect(getCapabilitiesForModel("kiro", "claude-opus-4-8-thinking").contextWindow).toBe(1000000);
+  });
+});