fix(workflows): reject divergent root history

[WyvernMonarch]

Summary

• validate the root workflow context before committing a completed result
• reject a renamed root step before registering or executing it when persisted history still contains an unvisited entry
• fail history divergence without replaying rollback against incompatible workflow code
• preserve removed(..., "message") migrations by marking generated message-history entries as visited

Testing

• pnpm run ci
• regression coverage in both yield and live modes verifies that a renamed root step throws HistoryDivergedError without mutating persisted history

[WyvernMonarch]

used workflows in my work, had an issue that required upstream changes, cause I had my stuff crashing cause of it

big fan of rivet!