fix(scripts): resolve check-migration-journal.mjs path via repoRoot, not undefined scriptsDir

master (fork/master @ 491c36f, landed via RBR-1033/#14) shipped
runMigrationJournalGuard() referencing an undefined 'scriptsDir'
variable in scripts/run-vitest-stable.mjs. This throws a ReferenceError
before any test suite starts, breaking every 'general' and 'serialized'
vitest invocation on master right now (confirmed live: general-server
shard 1/30 crashes immediately without this fix, succeeds with it).

The guard script (scripts/check-migration-journal.mjs) and its own
dedicated tests are correct; only this call site had the wrong
identifier — repoRoot was already defined and used everywhere else in
the file. One-line fix, no behavior change to the guard itself.

Verified on top of fork/master (efb5a78):
- general-server shard 1/30 (8 suites, 38 tests): now runs to completion
- packages/db/src/migration-journal-consistency.test.ts: 33/33 pass
- scripts/__tests__/check-migration-journal.test.mjs: 8/8 pass

Author: Paperclip-Paperclip

scripts/run-vitest-stable.mjs

@@ -254,7 +254,7 @@ function runMigrationJournalGuard() {
   // suite starts, so the operator sees the offending filename immediately
   // instead of hunting through skipped suites.
   console.log("\n[test:run] preflight: migration journal consistency");
-  const result = spawnSync(process.execPath, [path.join(scriptsDir, "check-migration-journal.mjs")], {
+  const result = spawnSync(process.execPath, [path.join(repoRoot, "scripts", "check-migration-journal.mjs")], {
     cwd: repoRoot,
     env: process.env,
     stdio: "inherit",