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/healthcheck.js

@@ -1,5 +1,5 @@
 'use strict';
-const ports = { reader: process.env.PORT || 8080, scraper: process.env.SCRAPER_PORT || 8091, sentinel: process.env.SENTINEL_PORT || 8092 };
+const ports = { reader: process.env.PORT || 8080, scraper: process.env.SCRAPER_PORT || 8091 };
 const service = process.argv[2] || 'reader';
 fetch(`http://127.0.0.1:${ports[service]}/health`, { signal: AbortSignal.timeout(4000) })
   .then(async r => { const body = await r.json(); if (!r.ok || !body.ok) process.exitCode = 1; }).catch(() => { process.exitCode = 1; });