feat(schedules): run jshvn/apartments every 4 hours, jittered

[jshvn]

What

• New every4h slot, 2 */4 * * * UTC, replacing hourly. hourly had no targets; with every4h it would have been a sixth slot, and the tests hold SLOTS at five, the free plan's cron budget.
• JITTER: a slot can wait a random [0, n) minutes before dispatching. every4h waits up to 30, so each run starts between :02 and :32, about 15 minutes either side of :17. The delay is drawn inside step.do, so a restarted instance keeps its draw, and slept with step.sleep, which costs no CPU. That adds two steps per firing.
• Registers jshvn/apartments watch.yml in every4h. It pings no healthcheck by choice: failed runs comment on its report issue. The rule in schedules/index.ts and the README checklist now allow "or say what alerts instead".
• wrangler.jsonc regenerated by task crons: 17 5 * * * and 2 */4 * * *, 2 of 5.

Before merging

• The GitHub App's installation on jshvn must include jshvn/apartments (Settings -> Applications -> Installed GitHub Apps). Otherwise every4h fails its step with App is not installed.
• Merging deploys to Cloudflare. A new cron can take up to 15 minutes to start firing.

Test plan

• task check: types, biome, 46 vitest tests, wrangler deploy --dry-run
• New test: a jittered slot runs pick a delay, then sleep jitter, then its dispatches, with the sleep under 30 minutes. It fails with JITTER emptied.
• Existing test: the unjittered evening slot runs only dispatch steps.
• jshvn/apartments watch.yml already verified with a manual workflow_dispatch (run 36074952086).
• After merge: task instances shows an every4h instance sleeping, then dispatching.