push-scope-guard write-side recovery tooling (captured-index disentangle)

Replace the interim PM-sanctioned reset/restore exception (pmmaster ruling 2026-08-17, after terra cross-lane work-capture) with tool-owned recovery. Guard v1.2 closed the read half: abort prints author/date/subject/diffstat per blocking SHA, run by the tool. The write half has no tool: separating another lane's captured work out of a local commit currently requires banned manual git (reset/restore), allowed only as a named on-channel PM-sanctioned exception. Owner lane: bin (sh.git); coordinate bin-venus-cc. Not on any critical path.