v2.8.5 — skill token intensity delta

Features

• Add avg tokens/run in Skill Detail modal.
• Add previous-period comparison for token intensity with direction hints (tốn hơn / tiết kiệm hơn).

Fixes

• Ensure Skill Detail previous-period window uses the same duration as current selected date range.

Docs

• None.

Chores

• Bump version to v2.8.5.