tip: tui reasoning header opacity (#51417)

[kvnloo]

Origin

• Upstream tip (read-only): fix(tui): honor thinking opacity on collapsed reasoning anomalyco/opencode#51417
• Base: dev
• github_writes_origin: 0

Intent

Honor theme.thinkingOpacity on collapsed completed reasoning headers; keep full warning while expanded/hovered/streaming.

Prove

cd packages/tui && bun test test/cli/tui/thinking.test.ts
# NEGATIVE (base product + tip tests): 0 pass / 1 fail (missing export)
# GREEN: 7 pass
# restore: 0 pass / 1 fail

Codex promote: parked.