fix(heartbeat): emit valid + even cron expressions on Linux/WSL

[oratis]

Closes #1.

What

secondsToCronApprox in src/heartbeat/install.ts is the Linux/WSL fallback for lisa heartbeat install --every <N> (macOS uses launchd StartInterval and never hits this path). The previous implementation produced cron lines that were either rejected by cron (*/90 * * * *, 0 */25 * * *) or run on an uneven schedule (0 */5 * * * fires at 00 05 10 15 20 then jumps back to 00 — only 4h between the last run and the next).

Change

Snap to safe field divisors:

Input	Before	After
--every 5h	0 */5 * * * (invalid wrap-around)	0 */4 * * *
--every 7h	0 */7 * * * (invalid wrap-around)	0 */6 * * *
--every 90m	*/90 * * * * (rejected by cron)	0 * * * *
--every 25h	0 */25 * * * (rejected by cron)	0 0 * * * (daily)
--every 8d	(would loop forever in old loop)	0 0 * * 0 (weekly)

When a snap occurs, the installer's printed instructions now include a one-line note like:

Note: requested 5 hours doesn't map cleanly to a single cron expression
(cron can't span midnight evenly with hour-divisors other than 1,2,3,4,6,8,12).
Snapped to the nearest safe divisor: 4 hours.

Verification

Ran a behavior table over 15 input values — all outputs are now syntactically valid cron and produce even intervals across day boundaries. Output table is in the issue and the commit message.

Scope

• One file: src/heartbeat/install.ts
• Pure internal change to one function (secondsToCronApprox) and its one caller (Linux branch of installHeartbeat)
• No public API change, no new deps
• macOS launchd path untouched
• npm run typecheck clean, npm run build clean

🤖 Generated with Claude Code