feat: implement rebase_skip with external abort detection and chain validation

- Add rebase_skip(): abort git rebase, restore branch from saved OID,
  mark as Skipped, resume from next pending branch
- Add external git rebase --abort detection in rebase_continue():
  compare branch OID with original_ref to detect bypassed aborts
- Add chain modification validation in both --continue and --skip:
  warn and skip branches that were deleted during chain rebase
- Route --skip flag in command handler
- Update conflict error messages to mention --skip option

Author: dashed

src/commands.rs

@@ -260,6 +260,8 @@ pub fn run(arg_matches: ArgMatches) -> Result<(), Error> {
         ("rebase", Some(sub_matches)) => {
             if sub_matches.is_present("continue_rebase") {
                 git_chain.rebase_continue()?;
+            } else if sub_matches.is_present("skip_rebase") {
+                git_chain.rebase_skip()?;
             } else if sub_matches.is_present("abort_rebase") {
                 git_chain.rebase_abort()?;
             } else {