Entries carried over from a previous session are now tagged sessionTail and can be filtered out

What

Entries that come from a prior session (rather than the current one) are now tagged with a sessionTail marker in several places:

• Recent session/path history entries get sessionTail: true when they are added to the result list.
• Entries appended to available-model lists (both plain string values and custom-model entries) are tagged sessionTail: true as well, with matching logic switched to a new helper.
• The model-picker state now also returns basePickerIds, a version of the picker id list with sessionTail-flagged entries filtered out, alongside the existing full pickerIds.

Why

This lets parts of the UI, like the base model picker, show only entries that belong to the current session instead of ones carried over from an earlier one, while still keeping the full list available where needed.