fix(shared): handle Windows rename-over-existing in write-file-atomically (#3222)

On Windows, renameSync fails with EPERM/EACCES when the target file
already exists. Fall back to unlink + rename on Windows permission
errors while preserving atomic semantics on other platforms.

🤖 Generated with OhMyOpenCode assistance
https://github.com/code-yeongyu/oh-my-opencode

src/shared/write-file-atomically.test.ts

@@ -0,0 +1,54 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test"
+import { readFileSync, writeFileSync, existsSync, rmSync, mkdirSync } from "fs"
+import { join } from "path"
+import { tmpdir } from "os"
+import { writeFileAtomically } from "./write-file-atomically"
+
+const testDir = join(tmpdir(), "write-file-atomically-test-" + Date.now())
+
+beforeEach(() => {
+  mkdirSync(testDir, { recursive: true })
+})
+
+afterEach(() => {
+  rmSync(testDir, { recursive: true, force: true })
+})
+
+describe("writeFileAtomically", () => {
+  it("writes content to a new file", () => {
+    // given
+    const filePath = join(testDir, "new-file.txt")
+    const content = "hello world"
+
+    // when
+    writeFileAtomically(filePath, content)
+
+    // then
+    expect(existsSync(filePath)).toBe(true)
+    expect(readFileSync(filePath, "utf-8")).toBe(content)
+  })
+
+  it("#given target file exists #when writeFileAtomically called #then overwrites successfully", () => {
+    // given
+    const filePath = join(testDir, "existing-file.txt")
+    const originalContent = "original content"
+    const newContent = "new content"
+    writeFileSync(filePath, originalContent, "utf-8")
+
+    // when
+    writeFileAtomically(filePath, newContent)
+
+    // then
+    expect(existsSync(filePath)).toBe(true)
+    expect(readFileSync(filePath, "utf-8")).toBe(newContent)
+    expect(existsSync(`${filePath}.tmp`)).toBe(false)
+  })
+
+  it("#given parent directory does not exist #when writeFileAtomically called #then throws", () => {
+    // given
+    const filePath = join(testDir, "nonexistent", "deep", "file.txt")
+
+    // when/then
+    expect(() => writeFileAtomically(filePath, "content")).toThrow()
+  })
+})