complete.path and complete.slash ran inline on the tui_gateway stdin
reader thread. complete.path spawns git ls-files and fuzzy-ranks the
whole repo; complete.slash does first-call prompt_toolkit imports plus a
skill-dir scan. While either ran, prompt.submit / session.interrupt sat
unread in the stdin pipe, freezing the TUI until the 120s RPC timeout
fired — most reliably reproduced by typing @ on a large repo / WSL2 mount.
Add both to _LONG_HANDLERS so completion runs on the existing thread
pool (write_json is already _stdout_lock-guarded). Root-cause fix:
covers any slow completion, not just the bare-@ trigger.
Fixes#21123
a94f657a50