feat: update Claude model to Opus 4.6 and enhance adaptive thinking support

- Changed model identifier from `claude-opus-4-5-20251101` to `claude-opus-4-6` across various files, including documentation and code references.
- Updated the SDK to support adaptive thinking for Opus 4.6, allowing the model to determine its own reasoning depth.
- Enhanced the thinking level options to include 'adaptive' and adjusted related components to reflect this change.
- Updated tests to ensure compatibility with the new model and its features.

These changes improve the model's capabilities and user experience by leveraging adaptive reasoning.

apps/server/tests/unit/lib/model-resolver.test.ts

@@ -35,7 +35,7 @@ describe('model-resolver.ts', () => {
 
     it("should resolve 'opus' alias to full model string", () => {
       const result = resolveModelString('opus');
-      expect(result).toBe('claude-opus-4-5-20251101');
+      expect(result).toBe('claude-opus-4-6');
       expect(consoleSpy.log).toHaveBeenCalledWith(
         expect.stringContaining('Migrated legacy ID: "opus" -> "claude-opus"')
       );
@@ -117,7 +117,7 @@ describe('model-resolver.ts', () => {
   describe('getEffectiveModel', () => {
     it('should prioritize explicit model over session and default', () => {
       const result = getEffectiveModel('opus', 'haiku', 'gpt-5.2');
-      expect(result).toBe('claude-opus-4-5-20251101');
+      expect(result).toBe('claude-opus-4-6');
     });
 
     it('should use session model when explicit is not provided', () => {