fix(tui): clarify unavailable reasoning summaries

[opencode-agent]

Summary

• keep metadata-only reasoning visually consistent as an expandable Thought · <duration> row
• show a clearly OpenCode-authored empty state when no readable reasoning summary is available
• use the info color instead of the assistant reasoning renderer so the status cannot be mistaken for model output

Testing

• bun test test/cli/tui/thinking.test.ts (5 passed)
• deterministic Anthropic redacted_thinking fixture in the real TUI, verified collapsed and expanded states
• bun typecheck attempted; tsgo crashed in the shared host environment
• full TUI suite: 189 passed, 1 skipped, 4 unrelated lifecycle/diff-viewer failures

Requested by: @rekram1-node (Aiden via Slack)