[1 of 7] Add thread settings to UserInput

[etraut-openai]

Stack position: [1 of 7]

Summary

The first three PRs in this stack are a cleanup pass before the actual thread settings API work.

Today, core has several overlapping "user input" ops: UserInput, UserInputWithTurnContext, and UserTurn. They differ mostly in how much next-turn state they carry, which makes the later queued thread settings update harder to reason about and review.

This PR starts that cleanup by adding the shared ThreadSettingsOverrides payload and allowing Op::UserInput to carry it. Existing variants remain in place here, so this layer is mostly a behavior-preserving API shape change plus mechanical constructor updates.

End State After PR3

By the end of PR3, Op::UserInput is the only "user input" core op. It can carry optional thread settings overrides for callers that need to update stored defaults with a turn, while callers without updates use empty settings. Op::UserInputWithTurnContext and Op::UserTurn are deleted.

End State After PR5

By the end of PR5, core will have only two ops for this area:

• Op::UserInput for user-input-bearing submissions.
• Op::ThreadSettings for settings-only updates.

Stack

1. [1 of 7] Add thread settings to UserInput (this PR)
2. [2 of 7] Remove UserInputWithTurnContext
3. [3 of 7] Remove UserTurn
4. [4 of 7] Placeholder for OverrideTurnContext cleanup
5. [5 of 7] Replace OverrideTurnContext with ThreadSettings
6. [6 of 7] Add app-server thread settings API
7. [7 of 7] Sync TUI thread settings