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>

Dockerfile

@@ -10,7 +10,6 @@ COPY discovery ./discovery
 COPY tools ./tools
 COPY scraper ./scraper
 COPY peering ./peering
-COPY sentinel ./sentinel
 COPY runtime ./runtime
 RUN mkdir -p /library /data && chown node:node /data /library
 ENV PORT=8080 LIBRARY=/library DATA=/data TZ=Europe/Berlin