[codex] Improve hybrid task checkbox rendering

[bendsp]

Summary

• draw a visible checkmark for checked hybrid task list items
• reduce the filled-box ambiguity by using a focused checked-state outline
• add a subtle line-through treatment for completed task text

Validation

• pnpm lint
• pnpm typecheck
• pnpm test:unit

[Copilot]

Pull request overview

This PR refines the hybrid markdown task-list styling in the renderer to make checked items more visually distinct and less ambiguous across themes.

Changes:

• Adds a dedicated checkmark glyph for checked hybrid task list items via a ::after pseudo-element.
• Adjusts the checked checkbox styling to use a subtle outline instead of the prior filled-box inset treatment.
• Applies a muted line-through styling to completed task text, including tuned thickness and color.

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