Improve subagent contrast in TUI

[gabec-openai]

Summary

• raise contrast for subagent transcript labels and fallback states
• remove low-contrast dim styling from role tags and error details
• make the closed-agent picker dot readable in dark theme

Validation

• just fmt
• just fix -p codex-tui
• cargo test -p codex-tui

[Copilot]

Pull request overview

Improves readability/contrast of the multi-agent experience in the TUI by adjusting Ratatui span styling for agent labels, status summaries, and the agent picker’s closed/open indicator.

Changes:

• Adjust agent picker “closed” status dot to avoid low-contrast dark gray styling.
• Increase contrast of agent transcript labels (nickname/thread id/role) by updating colors and removing dim styling.
• Increase contrast for several status/fallback lines (e.g., “Pending init”, “Shutdown”, error previews, and “No agents completed yet”) and update corresponding style assertions in tests.

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