[1 of 7] Add turn context to UserInput

[etraut-openai]

Stack position: [1 of 7]

Summary

The first three PRs in this stack are a cleanup pass before the actual next-turn state API work.

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

This PR starts that cleanup by adding the shared TurnContextOverrides 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 turn-context overrides for callers that need per-turn state, while callers without overrides use an empty context. 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::TurnContext for context-only updates.

Stack

1. [1 of 7] Add turn context to UserInput (this PR)
2. [2 of 7] Remove UserInputWithTurnContext
3. [3 of 7] Remove UserTurn
4. [4 of 7] Remove core OverrideTurnContext op
5. [5 of 7] Add queued core turn context op
6. [6 of 7] Add app-server next-turn state API
7. [7 of 7] Sync TUI next-turn state