Fold the sentinel into the scraper and drop the sentinel service (BL-0046)

The stall watchdog now runs inside the scraper (ScraperService.checkStalls,
every 30 s from the work loop): same rule as before, one checkpoint-safe
restart, quarantine on the second stall, missing checkpoint is no evidence,
and paused time no longer counts. Stalls are recorded as scraper incidents
(failure.kind 'stall'). The private /api/recovery endpoint is gone.

Dropped with the service: reader/scraper-unreachable incidents (supervisor
and Docker handle crashes; the health page shows "Scraper: unreachable"),
duplicate job-failure incidents and the unused /metrics endpoint.
The stack is now reader + scraper (+ optional peer-gateway).
The supervisor test moved to runtime/test (npm run test:runtime stays).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/start-all.js

@@ -12,7 +12,7 @@ if (fs.existsSync(lock)) {
   catch (e) { if (e.code !== 'ESRCH') throw e; fs.unlinkSync(lock); }
 }
 fs.writeFileSync(lock, String(process.pid), { flag: 'wx' });
-const supervisors = ['reader', 'scraper', 'sentinel'].map(name => new Supervisor(name, { root }));
+const supervisors = ['reader', 'scraper'].map(name => new Supervisor(name, { root }));
 supervisors.forEach(service => service.start());
 const keepalive = setInterval(() => {}, 60000);
 let closing = false;