grep: parallel workers produce non-deterministic output ordering

[u84u]

Description

With parallel workers, aict grep returns matches in a different order run to run. For an agent-facing tool this breaks output stability (and prompt caching downstream).

Reproduce

aict grep "func" tools -r --workers 8 > g1.txt
aict grep "func" tools -r --workers 8 > g2.txt
cmp g1.txt g2.txt   # differs

Verified on windows/amd64 at master (2026-07-14).

Expected

Deterministic ordering regardless of worker count — e.g. collect per-file results and emit in walk order.

Found during pre-launch audit; replaces auto-generated issue #17.