fix: use unique temp paths in saveFile() to prevent parallel write collisions (#810) (#828)

When multiple tool calls (e.g. concurrent gsd_save_decision) target the
same markdown file, the deterministic .tmp suffix caused ENOENT on
rename() because one caller consumed the temp file before another could
rename it.

Replace the static `.tmp` suffix with a per-call random suffix so each
concurrent writer gets its own temp file. Also clean up orphaned temp
files on rename failure.

src/resources/extensions/gsd/files.ts

@@ -5,6 +5,7 @@
 
 import { promises as fs } from 'node:fs';
 import { dirname, resolve } from 'node:path';
+import { randomBytes } from 'node:crypto';
 import { resolveMilestoneFile, relMilestoneFile, resolveGsdRootFile } from './paths.js';
 import { milestoneIdSort, findMilestoneIds } from './guided-flow.js';
 
@@ -705,9 +706,19 @@ export async function saveFile(path: string, content: string): Promise<void> {
   const dir = dirname(path);
   await fs.mkdir(dir, { recursive: true });
 
-  const tmpPath = path + '.tmp';
+  // Use a unique temp path per call to avoid collisions when parallel
+  // tool calls target the same file (e.g. concurrent gsd_save_decision).
+  // rename() is atomic on POSIX, so last-writer-wins is correct for
+  // regenerate-from-DB writes.
+  const tmpPath = path + `.tmp.${randomBytes(4).toString("hex")}`;
   await fs.writeFile(tmpPath, content, 'utf-8');
-  await fs.rename(tmpPath, path);
+  try {
+    await fs.rename(tmpPath, path);
+  } catch (err) {
+    // Clean up orphaned temp file on rename failure
+    await fs.unlink(tmpPath).catch(() => {});
+    throw err;
+  }
 }
 
 export function parseRequirementCounts(content: string | null): RequirementCounts {