fix(sdk): add mock handlers for ignore files in initial-session-state test

The test "discovers project files automatically when projectFiles is undefined"
was failing because the mock filesystem did not handle .codebuffignore and
.manicodeignore files that are checked during project file discovery.

Added mock handlers in both readFile and exists methods.

Author: brandonkachen

sdk/src/__tests__/initial-session-state.test.ts

@@ -27,6 +27,12 @@ describe('Initial Session State', () => {
         if (path.includes('.gitignore')) {
           return 'node_modules/\n.git/'
         }
+        if (path.includes('.codebuffignore')) {
+          return ''
+        }
+        if (path.includes('.manicodeignore')) {
+          return ''
+        }
         throw new Error(`File not found: ${path}`)
       },
       readdir: async (path: string) => {
@@ -62,6 +68,8 @@ describe('Initial Session State', () => {
         }) as any,
       exists: async (path: string) => {
         if (path.includes('.gitignore')) return true
+        if (path.includes('.codebuffignore')) return true
+        if (path.includes('.manicodeignore')) return true
         if (path.includes('src')) return true
         if (path.includes('.git')) return true
         if (path.includes('knowledge.md')) return true