test: add regression test for sync-worktree skip current milestone

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/tests/sync-worktree-skip-current.test.ts

@@ -0,0 +1,65 @@
+/**
+ * Regression test for #3641 — syncWorktreeStateBack skips current milestone
+ *
+ * When syncing worktree state back to main, the current milestone being
+ * merged should be skipped. Its files are already in the milestone branch
+ * and copying them back would conflict with the squash merge.
+ *
+ * The fix adds a `mid === milestoneId` skip guard inside the milestone
+ * iteration loop in syncWorktreeStateBack.
+ */
+
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+const src = readFileSync(
+  resolve(process.cwd(), 'src', 'resources', 'extensions', 'gsd', 'auto-worktree.ts'),
+  'utf-8',
+)
+
+describe('syncWorktreeStateBack skips current milestone (#3641)', () => {
+  it('syncWorktreeStateBack function exists', () => {
+    assert.ok(
+      src.includes('function syncWorktreeStateBack('),
+      'syncWorktreeStateBack function must be defined',
+    )
+  })
+
+  it('mid === milestoneId skip guard exists in the milestone loop', () => {
+    // Find syncWorktreeStateBack
+    const fnStart = src.indexOf('function syncWorktreeStateBack(')
+    assert.ok(fnStart !== -1)
+
+    // Get a reasonable portion of the function
+    const fnBlock = src.slice(fnStart, fnStart + 3000)
+
+    // Find the for loop iterating milestones
+    const loopIdx = fnBlock.indexOf('for (const mid of wtMilestones)')
+    assert.ok(loopIdx !== -1, 'milestone iteration loop must exist')
+
+    // After the loop, there should be the skip guard
+    const loopBody = fnBlock.slice(loopIdx, loopIdx + 300)
+    assert.ok(
+      loopBody.includes('mid === milestoneId'),
+      'mid === milestoneId skip guard must exist inside the milestone loop',
+    )
+    assert.ok(
+      loopBody.includes('continue'),
+      'skip guard must use continue to skip the current milestone',
+    )
+  })
+
+  it('syncMilestoneDir is still called for non-current milestones', () => {
+    const fnStart = src.indexOf('function syncWorktreeStateBack(')
+    assert.ok(fnStart !== -1)
+
+    const fnBlock = src.slice(fnStart, fnStart + 3000)
+
+    assert.ok(
+      fnBlock.includes('syncMilestoneDir('),
+      'syncMilestoneDir must still be called for other milestones',
+    )
+  })
+})