feat(refinery): implement handleSuccess for merge queue

Implement success handling for the merge queue Engineer:
- Add handleSuccess method that handles successful merge completion
- Update MR body with merge_commit SHA and close_reason
- Close MR with 'merged' reason
- Close source issue with reference to MR ID
- Delete source branch if delete_merged_branches is configured
- Add DeleteRemoteBranch method to git package
- Add git client to Engineer struct
- Add tests for new functionality

Closes gt-3x1.5

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/git/git.go

@@ -347,7 +347,7 @@ func (g *Git) RemoteBranchExists(remote, branch string) (bool, error) {
 	return out != "", nil
 }
 
-// DeleteBranch deletes a branch.
+// DeleteBranch deletes a local branch.
 func (g *Git) DeleteBranch(name string, force bool) error {
 	flag := "-d"
 	if force {
@@ -357,6 +357,12 @@ func (g *Git) DeleteBranch(name string, force bool) error {
 	return err
 }
 
+// DeleteRemoteBranch deletes a branch from the remote.
+func (g *Git) DeleteRemoteBranch(remote, branch string) error {
+	_, err := g.run("push", remote, "--delete", branch)
+	return err
+}
+
 // Rev returns the commit hash for the given ref.
 func (g *Git) Rev(ref string) (string, error) {
 	return g.run("rev-parse", ref)