* fix(cli): handle spaces in @file mentions
The @file parser truncated at the first space, so dragging or pasting a
filename like `@PREPING_ Building Agent.pdf` only matched `@PREPING_`
and warned "file not found". Now supports `@"..."` / `@'...'` quoted
form for explicit paths, plus a greedy expansion fallback that walks
across whitespace until an existing file resolves (bounded by newlines,
the next `@`, and a 20-token cap). Autocomplete also returns quoted
mentions for any candidate containing a space.
* style: apply ruff format to file_mentions
Ziheng Zhang
7f1aa3b0f6