Fix: TUI single-line paste cursor position

[Octane0411]

Fix: TUI single-line paste cursor position

Description

This PR fixes a bug where pasting single-line text in the TUI prompt would incorrectly move the cursor to the end of the input buffer, instead of placing it immediately after the pasted text.

Issue

Fixes #7210

Changes

• Removed input.gotoBufferEnd() from the onPaste handler in opencode/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx.

Verification

• Verified that input.gotoBufferEnd() was the cause of the cursor jump.
• The default behavior of the textarea component handles cursor positioning correctly for pasted text.
• Retained markDirty() and requestRender() to ensure UI updates correctly after paste.