fix(tui): preserve session prompt draft during permissions

[kitlangton]

Summary

• keep the session prompt mounted while permission or question prompts are active so draft input is preserved
• continue using the existing visible and disabled prompt props instead of conditionally unmounting the prompt
• fix the regression introduced by the TUI prompt slot refactor where permission prompts cleared in-progress input

Testing

• manually ran the local TUI with bun run --cwd packages/opencode dev
• reproduced the permission prompt flow and verified typed draft text remains after accepting the permission modal
• bun typecheck in packages/opencode is currently blocked by unrelated existing workspace errors