Add support for an additional exclusion file besides .gitignore and .geminiignore (#16487)

Co-authored-by: Adam Weidman <adamfweidman@google.com>

packages/cli/src/config/config.test.ts

@@ -126,10 +126,12 @@ vi.mock('@google/gemini-cli-core', async () => {
     DEFAULT_MEMORY_FILE_FILTERING_OPTIONS: {
       respectGitIgnore: false,
       respectGeminiIgnore: true,
+      customIgnoreFilePaths: [],
     },
     DEFAULT_FILE_FILTERING_OPTIONS: {
       respectGitIgnore: true,
       respectGeminiIgnore: true,
+      customIgnoreFilePaths: [],
     },
     createPolicyEngineConfig: vi.fn(async () => ({
       rules: [],
@@ -704,6 +706,9 @@ describe('loadCliConfig', () => {
     expect(config.getFileFilteringRespectGeminiIgnore()).toBe(
       DEFAULT_FILE_FILTERING_OPTIONS.respectGeminiIgnore,
     );
+    expect(config.getCustomIgnoreFilePaths()).toEqual(
+      DEFAULT_FILE_FILTERING_OPTIONS.customIgnoreFilePaths,
+    );
     expect(config.getApprovalMode()).toBe(ApprovalMode.DEFAULT);
   });