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>

docs/peering.md

@@ -58,7 +58,7 @@ past one instance for the same usage-counter directory.
    Tailscale TCP forward is recommended for the pilot; the default loopback binding
    is not remotely reachable. Do not expose port 8093 on the public internet merely
    to make pairing work. Use a stable tailnet DNS name, not a changing IP. Keep
-   scraper (8091) and Sentinel (8092) private.
+   the scraper (8091) private.
 
 The `PEER_BIND`, `PEER_PORT`, `PEER_DATA_PATH`, and `PEER_USAGE_PATH` Compose variables
 control the bind and persistent paths. The gateway's internal port remains 8093.