File path autocomplete regression: @ symbol only works at start of prompt, not mid-prompt

Description

File path completion has regressed in usability. Previously, tab completion for file paths worked anywhere in a prompt. Now, the @ symbol for file path autocomplete only works at the start of a prompt, not in the middle.

Expected Behavior

Users should be able to trigger file path autocomplete anywhere in their prompt, for example:

• "Can you update the function in @src/utils/helper.js to handle errors better"
• "Refactor @components/Button.tsx and @components/Input.tsx to use a shared theme"

Current Behavior

The @ symbol for file path autocomplete only works at the beginning of a prompt, not mid-prompt. This forces users to either:

1. Put all file references at the start of their message
2. Type out full paths manually without autocomplete

Impact

This is a step backward in terms of usability and workflow flexibility compared to the previous tab-based completion that worked anywhere in the prompt.

Suggested Fix

Enable @ symbol file path autocomplete to work anywhere in the prompt text, not just at the start.