feat(soak): scripts/seed-accounts.ts CLI wrapper

Thin standalone entry for pre-seeding N accounts before the first
harness run. Wraps SessionPool.seed and writes .env.stresstest.

End-to-end verified: ran against local dev with --count=4, all 4
accounts landed in the DB with is_test_account=TRUE, cred file
written with correct format.

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

tests/soak/scripts/seed-accounts.ts

@@ -0,0 +1,60 @@
+#!/usr/bin/env tsx
+/**
+ * Seed N soak-harness accounts via the register endpoint.
+ *
+ * Usage:
+ *   TEST_URL=http://localhost:8000 \
+ *   SOAK_INVITE_CODE=SOAKTEST \
+ *     bun run seed -- --count=16
+ *
+ * The invite code must already exist and be flagged marks_as_test=TRUE
+ * in the target environment. See docs/soak-harness-bringup.md.
+ */
+
+import * as path from 'path';
+import { SessionPool } from '../core/session-pool';
+import { createLogger } from '../core/logger';
+
+function parseArgs(argv: string[]): { count: number } {
+  const result = { count: 16 };
+  for (const arg of argv.slice(2)) {
+    const m = arg.match(/^--count=(\d+)$/);
+    if (m) result.count = parseInt(m[1], 10);
+  }
+  return result;
+}
+
+async function main(): Promise<void> {
+  const { count } = parseArgs(process.argv);
+  const targetUrl = process.env.TEST_URL ?? 'http://localhost:8000';
+  const inviteCode = process.env.SOAK_INVITE_CODE;
+  if (!inviteCode) {
+    console.error('SOAK_INVITE_CODE env var is required');
+    console.error('  Local dev: SOAK_INVITE_CODE=SOAKTEST');
+    console.error('  Staging:   SOAK_INVITE_CODE=5VC2MCCN');
+    process.exit(2);
+  }
+
+  const credFile = path.resolve(__dirname, '..', '.env.stresstest');
+  const logger = createLogger({ runId: `seed-${Date.now()}` });
+
+  logger.info('seed_start', { count, targetUrl, credFile });
+  try {
+    const accounts = await SessionPool.seed({
+      targetUrl,
+      inviteCode,
+      count,
+      credFile,
+      logger,
+    });
+    logger.info('seed_complete', { created: accounts.length });
+    console.error(`Seeded ${accounts.length} accounts → ${credFile}`);
+  } catch (err) {
+    logger.error('seed_failed', {
+      error: err instanceof Error ? err.message : String(err),
+    });
+    process.exit(1);
+  }
+}
+
+main();