Clicking a skill shortcut replaces any text already typed in the input box, instead of appending to it.

Affected components:

• GuidedPromptStarter.tsx (new session page)
• SkillShortcutsPanel.tsx (existing conversation page)

Both call setInput(value) directly, discarding any existing input.

Steps to reproduce:

1. Type some text in the input box
2. Click a skill shortcut button
3. The typed text is replaced by the skill prompt

Expected: The skill prompt is appended to the existing text.