Refactor: move filterByMinDuration into TimeEntryCollection and make grouped() return a collection

[igancev]

Tasks

• Move WorkReportCommand::filterByMinDuration() logic into TimeEntryCollection as a public, immutable method returning a new TimeEntryCollection.
• Change TimeEntryCollection::grouped() to return TimeEntryCollection instead of TimeEntry[], keeping it immutable (no mutation of $this->items).
• Update WorkReportCommand and any other callers to consume the new APIs.
• Adjust/extend tests to cover the new return types and behavior.

Files

• src/TimeEntryCollection.php
• src/Cli/WorkReportCommand.php
• tests/ — relevant tests for both classes