feat(sling): register --epic and --convoy flags

Add flag variable declarations and Cobra flag registrations for:
- --epic: link auto-created convoy to parent epic
- --convoy: add to existing convoy instead of creating new

Closes: gt-n3o

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

internal/cmd/sling.go

@@ -97,6 +97,8 @@ var (
 	slingAgent    string // --agent: override runtime agent for this sling/spawn
 	slingNoConvoy bool   // --no-convoy: skip auto-convoy creation
 	slingNoMerge  bool   // --no-merge: skip merge queue on completion (for upstream PRs/human review)
+	slingEpic     string // --epic: link auto-created convoy to parent epic
+	slingConvoy   string // --convoy: add to existing convoy instead of creating new
 )
 
 func init() {
@@ -115,6 +117,8 @@ func init() {
 	slingCmd.Flags().BoolVar(&slingNoConvoy, "no-convoy", false, "Skip auto-convoy creation for single-issue sling")
 	slingCmd.Flags().BoolVar(&slingHookRawBead, "hook-raw-bead", false, "Hook raw bead without default formula (expert mode)")
 	slingCmd.Flags().BoolVar(&slingNoMerge, "no-merge", false, "Skip merge queue on completion (keep work on feature branch for review)")
+	slingCmd.Flags().StringVar(&slingEpic, "epic", "", "Link auto-created convoy to parent epic")
+	slingCmd.Flags().StringVar(&slingConvoy, "convoy", "", "Add to existing convoy instead of creating new")
 
 	rootCmd.AddCommand(slingCmd)
 }