feat(refinery): implement merge execution with config and retry logic

Implements gt-3x1.3: Merge execution (merge, test, push)

Changes:
- Add MergeConfig struct with run_tests, test_command, delete_merged_branches,
  push_retry_count, and push_retry_delay_ms configuration options
- Add DefaultMergeConfig() with sensible defaults (tests enabled, go test ./...,
  branch cleanup, 3 retries with 1s base delay)
- Update ProcessMR to use MergeConfig for all settings
- Add pushWithRetry() with exponential backoff for transient failures
- Add gitOutput() helper to get command stdout (for merge commit SHA)
- Return merge commit SHA in MergeResult on success
- Conditional branch deletion based on config.DeleteMergedBranches

Configuration (in .gastown/config.json):
{
  "merge_queue": {
    "run_tests": true,
    "test_command": "go test ./...",
    "delete_merged_branches": true,
    "push_retry_count": 3,
    "push_retry_delay_ms": 1000
  }
}

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

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

internal/refinery/types.go

@@ -111,6 +111,41 @@ const (
 )
 
 
+// MergeConfig contains configuration for the merge process.
+type MergeConfig struct {
+	// RunTests controls whether tests are run after merge.
+	// Default: true
+	RunTests bool `json:"run_tests"`
+
+	// TestCommand is the command to run for testing.
+	// Default: "go test ./..."
+	TestCommand string `json:"test_command"`
+
+	// DeleteMergedBranches controls whether merged branches are deleted.
+	// Default: true
+	DeleteMergedBranches bool `json:"delete_merged_branches"`
+
+	// PushRetryCount is the number of times to retry a failed push.
+	// Default: 3
+	PushRetryCount int `json:"push_retry_count"`
+
+	// PushRetryDelayMs is the base delay between push retries in milliseconds.
+	// Each retry doubles the delay (exponential backoff).
+	// Default: 1000
+	PushRetryDelayMs int `json:"push_retry_delay_ms"`
+}
+
+// DefaultMergeConfig returns the default merge configuration.
+func DefaultMergeConfig() MergeConfig {
+	return MergeConfig{
+		RunTests:             true,
+		TestCommand:          "go test ./...",
+		DeleteMergedBranches: true,
+		PushRetryCount:       3,
+		PushRetryDelayMs:     1000,
+	}
+}
+
 // RefineryStats contains cumulative refinery statistics.
 type RefineryStats struct {
 	// TotalMerged is the total number of successful merges.