gate: quote mask glues a backslash-newline onto the previous word, hiding the next word from the subagent guard #306

Found by the 2026-10-02 differential fuzz sweep over the gate fixes merged on 2026-10-01 (residue of GH #286).

In hooks/gates/_quotemask.py (about lines 120-130) a backslash-newline is blanked only when at_word_start is true. Directly after a word character it is masked as two Q characters and glued to the previous word, even when whitespace follows. So the next word is hidden. In a subagent, plain stash, reset and clean are then allowed when the continuation sits: after the command word, mid-word in the git word or the subcommand, after env/time/sudo/command/eval, after an open brace, then or do, or trailing at the end. The same shapes inside a bash -c or eval body also allow. A space before the backslash denies correctly.

Reproducers (one file per shape) are in the sweep's scratchpad under zfz/repro/ with names starting 286-g1-; the maintainer has them. Fix direction: a continuation glues to the previous word only when the next character is not whitespace or a separator; keep the three mask copies and tests/hooks/quotemask_drift.py in sync. Differential-fuzz it per docs/reference/repo-gotchas.md.