Pre/post command hooks

git-spice should support hooks that it runs before/after certain operations
like post-restack, post-submit, etc.
This would allow power users to build custom workflows on top of git-spice
operations.

Refs https://github.com/abhinav/git-spice/issues/414#issuecomment-2672423604

This issue only tracks the high-level design of the hooks system,
not a comprehensive list of hooks.
One supported hook will be enough to resolve this issue.

Open questions:

- Where do hooks go? .git/hooks/spice/ or .git/spice/hooks/?
- Do we want to support in-repo hooks?
- How (if at all) should we support the core.hooksPath configuration option?