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/prepare-compose.js

@@ -2,7 +2,7 @@
 'use strict';
 const fs = require('fs'), path = require('path'), crypto = require('crypto');
 const root = path.resolve(__dirname, '..');
-for (const relative of ['data', 'data/scraper', 'data/sentinel', 'library', 'backup', '.secrets']) fs.mkdirSync(path.join(root, relative), { recursive: true });
+for (const relative of ['data', 'data/scraper','library', 'backup', '.secrets']) fs.mkdirSync(path.join(root, relative), { recursive: true });
 const file = path.join(root, '.secrets', 'scraper_token');
 if (!fs.existsSync(file)) fs.writeFileSync(file, crypto.randomBytes(32).toString('hex') + '\n', { flag: 'wx', mode: 0o600 });
 if (!fs.existsSync(path.join(root, '.env'))) fs.copyFileSync(path.join(root, '.env.example'), path.join(root, '.env'));