feat: implement gt mq retry command for failed merge requests

Add 'gt mq retry <rig> <mr-id>' command to retry failed MRs:
- Added GetMR() and Retry() methods to refinery.Manager
- Added RegisterMR() for persistent MR tracking
- Added PendingMRs field to Refinery state
- Created new mq.go command file with retry subcommand
- Support --now flag for immediate processing
- Added comprehensive tests for retry functionality

Closes gt-svi.4

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

internal/refinery/types.go

@@ -38,6 +38,10 @@ type Refinery struct {
 	// CurrentMR is the merge request currently being processed.
 	CurrentMR *MergeRequest `json:"current_mr,omitempty"`
 
+	// PendingMRs tracks merge requests that have been submitted.
+	// Key is the MR ID.
+	PendingMRs map[string]*MergeRequest `json:"pending_mrs,omitempty"`
+
 	// LastMergeAt is when the last successful merge happened.
 	LastMergeAt *time.Time `json:"last_merge_at,omitempty"`