test(intake): the second review pass, and CLAUDE.md for the rules it set

Six of the nine fixes have a test that fails against the previous tree: the
metadata row's currency, the Create-path hold, two concurrent batches booking
once, mrkdwn escaping including plain_text, the escaped currency, and
candidate-row currency.

Three do not, and the reason is the same each time — the window needs a crash
or a lost race that a test cannot force. Asserted instead: a stale metadataId
falls back to the live row rather than reporting a phantom write; two writes
racing to create a metadata row both resolve to one; the create path records a
baseline at all. All three passed before as well.

src/__tests__/integration/helpers.ts

@@ -23,6 +23,25 @@ export function mockDbWithPool(p: Pool) {
       const result = await p.query(sql, params);
       return result.rows[0] ?? null;
     },
+    // Mirrors withTransaction: one connection, BEGIN/COMMIT around the callback,
+    // so pg_advisory_xact_lock actually holds across the statements inside it.
+    withTransaction: async (fn: (q: unknown) => Promise<unknown>) => {
+      const client = await p.connect();
+      try {
+        await client.query("BEGIN");
+        const result = await fn({
+          queryRaw: async (sql: string, params: unknown[] = []) => (await client.query(sql, params)).rows,
+          queryRow: async (sql: string, params: unknown[] = []) => (await client.query(sql, params)).rows[0] ?? null,
+        });
+        await client.query("COMMIT");
+        return result;
+      } catch (e) {
+        await client.query("ROLLBACK");
+        throw e;
+      } finally {
+        client.release();
+      }
+    },
     prisma: p,
   }));
 }