Support multiple journal streams (e.g. work/personal)

[d12frosted]

Problem

vulpea-journal assumes a single template and a single timeline. Users who maintain separate work and personal journals need two parallel streams with independent navigation, calendar marking, and creation commands.

See d12frosted/vulpea#230 (reply in thread) for context. Key user requirements:

• Separate files for work and personal (privacy — screen sharing, potential encryption)
• Template selection when creating entries (DWIM from context, or prompt)
• Navigation (prev/next) scoped to the active stream
• Calendar/sidebar aware of which stream is active

Current state

The mechanical part works today via let-binding vulpea-journal-default-template with separate commands. But all interactive features (vulpea-journal-today, vulpea-journal-date, prev/next, calendar, sidebar widgets) assume a single template/timeline.

Design considerations

• DWIM approach: if context makes the stream clear (e.g. viewing a work journal entry), use that stream automatically; otherwise prompt
• Tag-based stream identification: streams could be distinguished by tags (e.g. journal+work vs journal+personal), which ties into potential broader tag-based namespace support in vulpea core
• Privacy: separate files per stream is a hard requirement for some users (screen sharing, encryption)