feat(handoffs): make follow-up atomic

[chicoxyzzy]

Summary

• add compare-and-set handoff update, status, and delete contracts across memory, SQLite, service, and MCP
• add an atomic, idempotent accept-with-follow-up command that never claims or launches work
• preserve snapshot-v1 proposal compatibility while requiring revisions for new assistant handoff edits
• document the intentional alpha contract changes and retry semantics

Verification

• make views
• go test ./...
• go vet ./...
• go test -race -count=1 ./...
• three independent agent reviews, all clean after fixes