[codex] Improve sidebar task title readability

[aandresalvarez]

Summary

• Remove the permanent expanded-workspace child guide and align child task row shades with workspace row shades.
• Render sidebar task titles as natural text instead of fixed head/tail fragments.
• Collapse exact repeated title halves in sidebar title normalization so duplicated stored titles read cleanly.

Why

The left sidebar was spending width on a vertical guide and extra indentation, leaving task titles cramped and harder to scan. Some task titles also rendered as repeated strings, which made the cramped row state look worse.

Validation

• swift test --filter FormattersTests
• swift test --filter TaskEventTimelineSidebarTests
• swift test --filter SidebarGroupingTests/leanSidebarPresentationContracts
• git diff --check
• ./script/build_and_run.sh --verify

Verified visually in ASTRA Dev.app after rebuild.