fix: use release start timestamp for delta zip filename

[maksimtech]

Problem

The delta zip filename was derived from new Date() inside
DeltaCommand.run(), which captures the time when the function
executes rather than when the release started.

If processing crosses an hour boundary the filename ends up one hour
ahead of the release tag (e.g. tag cve_2026-01-29_0200Z contains
a file named 2026-01-29_delta_CVEs_at_0300Z.zip).

Fixes #124.

Fix

Replace new Date() with new Date(options.start) so the filename
always reflects the release start time:

// Before
const timestamp = new Date();

// After
const timestamp = new Date(options.start);

Notes

The --yesterday-all branch already handles this correctly using
startOfYesterday() — this fix brings the default branch in line
with the same approach.

This fix is equivalent to the approach in PR #130.