Add an export/report command (standup text, markdown digest, HTML)

[kacheo]

Problem

devlog stores rich structured entries but can only render them to the terminal or --json. There's no way to produce a shareable report — e.g. a standup blurb, a weekly markdown digest, or an HTML page.

Proposal

Add devlog export (a reporting layer over show's existing range logic) with format options:

• --format standup — concise "what I did / blockers" text for a range
• --format markdown — a rolled-up digest across the range
• --format html — a self-contained HTML page

Note

A standup command existed and was removed in v0.1.0 in favor of show --from/--until. This issue proposes the broader, format-flexible successor rather than reviving standup as-is.