Monthly separators in jot list

**Description**

Visually group entries by month.

**Acceptance criteria**

- Separator format:

```ts
--- January 2026 ---
[2026-01-04 22:31] notion but in the terminal
....

--- February 2026 ---
[2026-01-06 09:12] onboarding tools assume users read
[2026-01-09 01:03] loneliness isn’t social, it’s unseen
...
```
- No impact on storage format
- Output-only change