fix(changelog): remap stale SHAs in-memory to survive ctx exit dump

[EspenAlbert]

Summary

_resolve_stale_shas detected stale SHAs after rebase and applied corrections via file-level regex replacement. But pkg_ctx.__exit__ unconditionally re-dumps self._actions (still holding old SHAs), overwriting the file fix. A secondary issue: the regex _SHORT_SHA_PATTERN couldn't handle YAML-quoted SHA values.

Switched to in-memory operations: apply_remap_to_actions mutates FixAction.short_sha directly on ctx._actions, and remove_actions_by_sha filters the list. When __exit__ dumps, it writes corrected SHAs. Removed dead file-level functions (apply_remap_to_file, remove_actions_from_file) and _SHORT_SHA_PATTERN.

Considerations

• apply_remap_to_actions mutates in-place (returns count); remove_actions_by_sha returns a new list. Callers must assign the result of remove_actions_by_sha back to ctx._actions