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: