refactor(ultraplan-local): extract atomicWriteJson to lib/util

Three changes in one commit:

1. NEW lib/util/atomic-write.mjs — exports atomicWriteJson(path, obj),
   the canonical tmp+rename pattern. Reused by pre-compact-flush.mjs and
   (in subsequent steps) by the new session-state writer.

2. NEW tests/lib/atomic-write.test.mjs — 4 unit tests covering
   round-trip, no-orphan-tmp, overwrite-atomic, pretty-print formatting.

3. REFACTOR hooks/scripts/pre-compact-flush.mjs — replace the inline
   atomicWrite() with the imported atomicWriteJson(). Also fixes a
   pre-existing syntax error (leading whitespace + stray --resume token
   outside the comment block) that silently broke the hook from v3.1.0
   onward — PreCompact runtime is fail-open and swallowed the error.
   File reformatted with standard zero-indent JS.

163 → 167 tests, 0 fail.

Step 2 of /ultracontinue v3.3.0 (project 2026-05-01-ultracontinue).

plugins/ultraplan-local/tests/lib/atomic-write.test.mjs

@@ -0,0 +1,61 @@
+// tests/lib/atomic-write.test.mjs
+// Unit tests for lib/util/atomic-write.mjs
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { mkdtempSync, rmSync, readFileSync, existsSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { atomicWriteJson } from '../../lib/util/atomic-write.mjs';
+
+test('atomicWriteJson — writes valid JSON and round-trips', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'aw-test-'));
+  try {
+    const path = join(dir, 'state.json');
+    const obj = { schema_version: 1, status: 'in_progress', items: [1, 2, 3] };
+    atomicWriteJson(path, obj);
+    const read = JSON.parse(readFileSync(path, 'utf-8'));
+    assert.deepEqual(read, obj);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('atomicWriteJson — leaves no .tmp orphan after success', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'aw-test-'));
+  try {
+    const path = join(dir, 'state.json');
+    atomicWriteJson(path, { ok: true });
+    assert.equal(existsSync(path), true);
+    assert.equal(existsSync(path + '.tmp'), false);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('atomicWriteJson — overwrites existing file atomically', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'aw-test-'));
+  try {
+    const path = join(dir, 'state.json');
+    writeFileSync(path, '{"old":true}');
+    atomicWriteJson(path, { new: true });
+    const read = JSON.parse(readFileSync(path, 'utf-8'));
+    assert.deepEqual(read, { new: true });
+    assert.equal(existsSync(path + '.tmp'), false);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('atomicWriteJson — pretty-prints with 2-space indent', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'aw-test-'));
+  try {
+    const path = join(dir, 'state.json');
+    atomicWriteJson(path, { a: 1, b: { c: 2 } });
+    const text = readFileSync(path, 'utf-8');
+    assert.match(text, /\n {2}"a": 1/);
+    assert.match(text, /\n {4}"c": 2/);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});