[BUG] @ autocomplete regression: lowercase queries no longer match uppercase paths

Description

Lowercase @ mention queries no longer match paths containing uppercase characters. This is a regression — it previously worked.

Reproduction

1. Have a directory structure with uppercase names, e.g. vox-systems-inc/agents/CEO/
2. Type @ceo in the input
3. Expected: vox-systems-inc/agents/CEO/ appears in autocomplete
4. Actual: No match

Similarly, @working does not match a directory named working/ if searched as @WORKING — but the primary issue is lowercase-to-uppercase (the common case).

Environment

• macOS (case-insensitive filesystem)
• Claude Code (latest version)

Context

This is related to but distinct from #14623 (closed, not planned). That issue reported inconsistent mixed-case matching. This issue reports a regression where case-insensitive matching that previously worked has stopped working. Lowercase queries matching uppercase paths is the most common real-world pattern (users type lowercase, paths often contain uppercase).