fix(noter): use the fs import instead of require in the heartbeat helpers (#3)

This module is ESM, so `require` is not defined at runtime. `node --check`
only parses, so it passed while both helpers would have thrown: the write
was swallowed by its catch, and the read fell through to `Infinity`, which
the watchdog reads as a permanently stale heartbeat — it would have exited
the process a minute after every scan and left the bot restarting in a loop.

`fs` is already imported at the top of the file; use it.

noter/bot.js

@@ -52,7 +52,7 @@ const STALE_AFTER = SCAN_INTERVAL * 3;
 
 function touchHeartbeat() {
   try {
-    require('fs').writeFileSync(HEARTBEAT_FILE, String(Date.now()));
+    fs.writeFileSync(HEARTBEAT_FILE, String(Date.now()));
   } catch (err) {
     log('WARN', 'Could not write heartbeat', { error: err.message });
   }
@@ -62,7 +62,7 @@ function touchHeartbeat() {
 /// yet — an unwritable or missing heartbeat counts as stale rather than healthy.
 function heartbeatAge() {
   try {
-    const t = Number(require('fs').readFileSync(HEARTBEAT_FILE, 'utf8'));
+    const t = Number(fs.readFileSync(HEARTBEAT_FILE, 'utf8'));
     return Number.isFinite(t) ? Date.now() - t : Infinity;
   } catch {
     return Infinity;