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)

internal/refinery/engineer.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/steveyegge/gastown/internal/beads"
+	"github.com/steveyegge/gastown/internal/git"
 	"github.com/steveyegge/gastown/internal/rig"
 )
 
@@ -67,6 +68,7 @@ func DefaultMergeQueueConfig() *MergeQueueConfig {
 type Engineer struct {
 	rig     *rig.Rig
 	beads   *beads.Beads
+	git     *git.Git
 	config  *MergeQueueConfig
 	workDir string
 
@@ -79,6 +81,7 @@ func NewEngineer(r *rig.Rig) *Engineer {
 	return &Engineer{
 		rig:     r,
 		beads:   beads.New(r.Path),
+		git:     git.NewGit(r.Path),
 		config:  DefaultMergeQueueConfig(),
 		workDir: r.Path,
 		stopCh:  make(chan struct{}),
@@ -256,12 +259,7 @@ func (e *Engineer) processOnce(ctx context.Context) error {
 
 	// 6. Handle result
 	if result.Success {
-		// Close with merged reason
-		reason := fmt.Sprintf("merged: %s", result.MergeCommit)
-		if err := e.beads.CloseWithReason(reason, mr.ID); err != nil {
-			fmt.Printf("[Engineer] Warning: failed to close MR %s: %v\n", mr.ID, err)
-		}
-		fmt.Printf("[Engineer] ✓ Merged: %s\n", mr.ID)
+		e.handleSuccess(mr, result)
 	} else {
 		// Failure handling (detailed implementation in gt-3x1.4)
 		e.handleFailure(mr, result)
@@ -305,6 +303,56 @@ func (e *Engineer) ProcessMR(ctx context.Context, mr *beads.Issue) ProcessResult
 	}
 }
 
+// handleSuccess handles a successful merge completion.
+// Steps:
+// 1. Update MR with merge_commit SHA
+// 2. Close MR with reason 'merged'
+// 3. Close source issue with reference to MR
+// 4. Delete source branch if configured
+// 5. Log success
+func (e *Engineer) handleSuccess(mr *beads.Issue, result ProcessResult) {
+	// Parse MR fields from description
+	mrFields := beads.ParseMRFields(mr)
+	if mrFields == nil {
+		mrFields = &beads.MRFields{}
+	}
+
+	// 1. Update MR with merge_commit SHA
+	mrFields.MergeCommit = result.MergeCommit
+	mrFields.CloseReason = "merged"
+	newDesc := beads.SetMRFields(mr, mrFields)
+	if err := e.beads.Update(mr.ID, beads.UpdateOptions{Description: &newDesc}); err != nil {
+		fmt.Printf("[Engineer] Warning: failed to update MR %s with merge commit: %v\n", mr.ID, err)
+	}
+
+	// 2. Close MR with reason 'merged'
+	if err := e.beads.CloseWithReason("merged", mr.ID); err != nil {
+		fmt.Printf("[Engineer] Warning: failed to close MR %s: %v\n", mr.ID, err)
+	}
+
+	// 3. Close source issue with reference to MR
+	if mrFields.SourceIssue != "" {
+		closeReason := fmt.Sprintf("Merged in %s", mr.ID)
+		if err := e.beads.CloseWithReason(closeReason, mrFields.SourceIssue); err != nil {
+			fmt.Printf("[Engineer] Warning: failed to close source issue %s: %v\n", mrFields.SourceIssue, err)
+		} else {
+			fmt.Printf("[Engineer] Closed source issue: %s\n", mrFields.SourceIssue)
+		}
+	}
+
+	// 4. Delete source branch if configured
+	if e.config.DeleteMergedBranches && mrFields.Branch != "" {
+		if err := e.git.DeleteRemoteBranch("origin", mrFields.Branch); err != nil {
+			fmt.Printf("[Engineer] Warning: failed to delete branch %s: %v\n", mrFields.Branch, err)
+		} else {
+			fmt.Printf("[Engineer] Deleted branch: %s\n", mrFields.Branch)
+		}
+	}
+
+	// 5. Log success
+	fmt.Printf("[Engineer] ✓ Merged: %s (commit: %s)\n", mr.ID, result.MergeCommit)
+}
+
 // handleFailure handles a failed merge request.
 // This is a placeholder that will be fully implemented in gt-3x1.4.
 func (e *Engineer) handleFailure(mr *beads.Issue, result ProcessResult) {

internal/refinery/engineer_test.go

@@ -200,6 +200,9 @@ func TestNewEngineer(t *testing.T) {
 	if e.beads == nil {
 		t.Error("expected beads client to be initialized")
 	}
+	if e.git == nil {
+		t.Error("expected git client to be initialized")
+	}
 	if e.config == nil {
 		t.Error("expected config to be initialized with defaults")
 	}
@@ -207,3 +210,11 @@ func TestNewEngineer(t *testing.T) {
 		t.Error("expected stopCh to be initialized")
 	}
 }
+
+func TestEngineer_DeleteMergedBranchesConfig(t *testing.T) {
+	// Test that DeleteMergedBranches is true by default
+	cfg := DefaultMergeQueueConfig()
+	if !cfg.DeleteMergedBranches {
+		t.Error("expected DeleteMergedBranches to be true by default")
+	}
+}