[BUG] @-mention file picker cannot descend into directory whose name contains a space — child files never suggested

Version: 2.1.227 (Claude Code), macOS (Darwin 25.5.0), terminal CLI.

Summary: @-mention file picker cannot descend into directory whose name contains space. Fuzzy match on partial segment works; once full segment plus / typed, child files never suggested — @-token terminates at space, rest of path becomes plain prompt text.

Repro:

1. Fixture:

mkdir -p "space test/dir with space" && echo x > "space test/dir with space/target file.ts"
mkdir -p plain-test/nospace && echo x > plain-test/nospace/targetfile.ts

1. Start claude, type @space — picker suggests space test/. Fuzzy match works.
2. Type @space test/ — no children listed. Expected: space test/dir with space/ suggested.
3. Type @space\ test/ — backslash escape also lists no children. No in-picker workaround.

Control: @plain-test/nospace/ lists plain-test/nospace/targetfile.ts — descent itself works for space-free paths.

Isolation: submitted-prompt parsing is fine — headless claude -p 'Print exact contents of @space\ test/dir\ with\ space/target\ file.ts' reads file correctly; unescaped fails as expected. Bug is picker tokenizer only.

Impact: any file under directory with space in name (e.g. ~/Prompts/git account update/) unreachable via @ picker.