Adds a region/function-scoped negative-grep idiom to the gsd-planner verification guidance plus a warn-only `validate_plan` check (`scanFileWideNegativeGateConflict`) that flags when a task's file-wide negative grep bans a construct a sibling task legitimately requires elsewhere in the same file. ReDoS-safe (linear, no RegExp on author patterns); region-scoped gates are exempt. Warn-only — never errors, never flips `valid`.
Closes#968
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>