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>

.env.example

@@ -7,7 +7,6 @@ READER_PORT=8080
 LIBRARY_PATH=./library
 DATA_PATH=./data
 SCRAPER_DATA_PATH=./data/scraper
-SENTINEL_DATA_PATH=./data/sentinel
 SCRAPER_TOKEN_PATH=./.secrets/scraper_token
 # Built-in backups of DATA_PATH (docs/backup.md): an existing folder, ideally on another
 # disk/NAS, writable by uid 1000. Unset = backups off (./backup is then mounted unused).