Add --start-date and --end-date support

The KernelCI dashboard allows users to select exact custom date ranges for issue listings. However, kci-dev results issues currently supports only --days.

As a result, dashboard-generated kci-dev commands must round custom ranges up to a whole-day duration. These commands cannot reproduce the exact start and end boundaries selected in the dashboard.

Add the following options to kci-dev results issues:

--start-date
--end-date

Requirements:

Accept YYYY-MM-DD dates and timezone-aware ISO 8601 timestamps.
Pass the corresponding exact boundaries to the dashboard API.
Require --start-date and --end-date to be used together.
Reject a range where the start is later than the end.
Make an explicit date range mutually exclusive with --days.
Preserve the existing --days behavior for backward compatibility.
Return clear usage errors for invalid or incomplete ranges.
Support the new options with both human-readable and JSON output.
Update command help, documentation, and examples.
Add pytest coverage for valid ranges, timestamps with timezones, invalid values, reversed ranges, incomplete pairs, and conflicts with --days.

Example:

kci-dev results issues \
--origin maestro \
--start-date 2026-08-01T00:00:00Z \
--end-date 2026-08-08T00:00:00Z

Once implemented, the KernelCI dashboard can generate commands that reproduce custom issue-listing date ranges without rounding or reporting omitted timestamp boundaries.

Start at the `kci-dev results issues` command and inspect how its existing `--days` option reaches the dashboard API. Add pytest coverage for the listed valid and invalid date-range cases, including both output formats. Done means paired, validated date options work with timezone-aware timestamps, conflicts and errors are clear, `--days` remains compatible, and help, documentation, and examples are updated.

Written by the indexing model from the issue text.