Prefer AGENTS.md over CLAUDE.md for project instructions (#439)

* Prefer AGENTS.md over CLAUDE.md for project instructions

* fix: preserve CLAUDE.md fallback behavior

* fix: isolate onboarding tests and preserve legacy init

* fix: restore full fsOperations exports in test mock and align compact cwd

* Fix onboarding test isolation and init migration guidance

* Tighten init prompt coverage and onboarding copy

* Handle nested project instruction paths consistently

* Fix NEW_INIT feature gate for Bun build

---------

Co-authored-by: 赵小落 <zhaoxiaoluo@zhaoxiaoluodeMac-mini.local>
Co-authored-by: zhaomo01 <zhaomo01@baidu.com>

src/utils/projectInstructions.ts

@@ -0,0 +1,55 @@
+import { dirname, join } from 'path'
+
+export const PRIMARY_PROJECT_INSTRUCTION_FILE = 'AGENTS.md'
+export const FALLBACK_PROJECT_INSTRUCTION_FILE = 'CLAUDE.md'
+
+export function getProjectInstructionFilePaths(dir: string): string[] {
+  return [
+    join(dir, PRIMARY_PROJECT_INSTRUCTION_FILE),
+    join(dir, FALLBACK_PROJECT_INSTRUCTION_FILE),
+  ]
+}
+
+export function getProjectInstructionFilePath(
+  dir: string,
+  existsSync: (path: string) => boolean,
+): string {
+  const [primaryPath, fallbackPath] = getProjectInstructionFilePaths(dir)
+  return existsSync(primaryPath)
+    ? primaryPath
+    : fallbackPath
+}
+
+export function hasProjectInstructionFile(
+  dir: string,
+  existsSync: (path: string) => boolean,
+): boolean {
+  return getProjectInstructionFilePaths(dir).some(path => existsSync(path))
+}
+
+export function findProjectInstructionFilePathInAncestors(
+  startDir: string,
+  existsSync: (path: string) => boolean,
+): string | null {
+  let currentDir = startDir
+
+  while (true) {
+    if (hasProjectInstructionFile(currentDir, existsSync)) {
+      return getProjectInstructionFilePath(currentDir, existsSync)
+    }
+
+    const parentDir = dirname(currentDir)
+    if (parentDir === currentDir) {
+      return null
+    }
+
+    currentDir = parentDir
+  }
+}
+
+export function isProjectInstructionFileName(name: string): boolean {
+  return (
+    name === PRIMARY_PROJECT_INSTRUCTION_FILE ||
+    name === FALLBACK_PROJECT_INSTRUCTION_FILE
+  )
+}