histedit: convert pick action into a class This converts the pick action into a histeditclass instance, as part of an ongoing effort to refactor histedit for maintainability and robustness. The test output changed because previously pick would only report the commit disappearing if there were no merge conflicts. Now that we've unified the normal and the --continue flows, they act the same and we get the warning even after --continue.

histedit: integrate action class into flow This takes the newly added histeditaction class and integrates it into the histedit run and bootstrapcontinue logic. No actions implement the class yet, but they will be add in upcoming patches.

histedit: add a new histeditaction class This adds a new class called histeditaction. It represents a single action in a histedit plan. Future patches will integrate it into the histedit flow, then convert each existing action in to the class form one by one. This is part of a larger refactor aimed at increasing histedit robustness, maintainability, and extensibility.