Notify TUI about plan mode prompts and user input requests

[etraut-openai]

Addresses #13478

Summary

• Add two new scopes for tui.notifications config: plan-mode-prompt and user-input-requested.
• Add Plan Mode prompt and user-input-requested notifications to the TUI so these events surface consistently outside of plan mode
• Add helpers and tests to ensure the new notification types publish the right titles, summaries, and type tags for filtering
• Add prioritization mechanism to fix an existing bug where one notification event could arbitrarily overwrite others

Testing

• Manually tested plan mode to ensure that notification appeared

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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