feat(tui): add approval command dialog

[T1mn]

What does this PR do?

Fixes #7726

• Adds a /approval command in the TUI with an allow/ask/reject dialog.
• Surfaces the command in / autocomplete and the command palette.
• Stores the session-level approval rule via session.update (SDK/OpenAPI synced).
• For longer flows, repeated “ask” prompts can break the rhythm and waste time; /approval lets users adjust permissions on the fly to keep work moving.

How did you verify your code works?

• Built and ran the TUI locally.

Allow:

Ask:

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Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

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The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[T1mn]

Many Codex CLI users are already used to /approval for permissions, so this keeps the transition to opencode smooth and familiar.
I’m not sure who will be reviewing this, if you spot anything that feels off, I’d really appreciate the feedback. I believe this should be a positive usability improvement, but I’m happy to adjust as needed.