Ask for update strategy instead of using merge as a default

[geofflamrock]

Using merge is a safe default update strategy, however might be a bit unexpected if someone uses the tool for the first time and doesn't really know this is going to happen, or has configured on a per-repo basis and forgets to do so for a new repo. There currently also isn't any logging to indicate which strategy is used, it just happens.

This PR changes the default to ask the user instead of using merge. It also provides examples of how to configure the setting for the future.

[Copilot]

Pull Request Overview

This PR changes the default branch update behavior to prompt the user instead of always merging, surfaces the chosen strategy in logs with configuration hints, and adds tests covering scenarios where no strategy is preconfigured.

• Prompt for update strategy when not specified or configured, with log messages and git-config examples.
• Refactor UpdateStack to return the chosen UpdateStrategy and update callers to use its result.
• Extend test suites to cover merge/rebase flows when no config exists and update README accordingly.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Stack/Commands/Remote/SyncStackCommand.cs	Capture returned strategy and compute forceWithLease solely from it
src/Stack/Commands/Helpers/StackHelpers.cs	Change UpdateStack to return UpdateStrategy, prompt user if unset, and log examples
src/Stack/Commands/Helpers/Questions.cs	Add SelectUpdateStrategy prompt constant
src/Stack.Tests/Commands/Stack/UpdateStackCommandHandlerTests.cs	Add tests for rebase/merge when no config exists
src/Stack.Tests/Commands/Remote/SyncStackCommandHandlerTests.cs	Add tests for prompting merge/rebase in remote sync when no config
README.md	Update default strategy description to reflect interactive prompt

Comments suppressed due to low confidence (3)

src/Stack/Commands/Helpers/StackHelpers.cs:501

• The signature of UpdateStack changed from void to returning UpdateStrategy. Please verify all other callers (outside of these two consumers) have been updated to handle the return value to avoid breaking callers.

    public static UpdateStrategy UpdateStack(

src/Stack.Tests/Commands/Stack/UpdateStackCommandHandlerTests.cs:668

• In the merge scenario test, stub inputProvider.Select(Questions.SelectUpdateStrategy, ...) to explicitly return UpdateStrategy.Merge so the test does not rely on the default enum value and remains robust.

        // Act

README.md:110

• [nitpick] Consider expanding this section to document that the tool logs the selected strategy and shows git config stack.update.strategy examples, so users know how to persist their choice.

You will be asked to select an update strategy if none is supplied or configured.