feat: add 'convoy' issue type with reactive completion (bd-hj0s)

- Add TypeConvoy to issue types for cross-project tracking
- Implement reactive completion: when all tracked issues close,
  convoy auto-closes with reason "All tracked issues completed"
- Uses 'tracks' dependency type (non-blocking, cross-prefix capable)
- Update help text for --type flag in list/create commands
- Add test for convoy reactive completion behavior

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/types/types.go

@@ -394,12 +394,13 @@ const (
 	TypeGate         IssueType = "gate"          // Async coordination gate
 	TypeAgent        IssueType = "agent"         // Agent identity bead
 	TypeRole         IssueType = "role"          // Agent role definition
+	TypeConvoy       IssueType = "convoy"        // Cross-project tracking with reactive completion
 )
 
 // IsValid checks if the issue type value is valid
 func (t IssueType) IsValid() bool {
 	switch t {
-	case TypeBug, TypeFeature, TypeTask, TypeEpic, TypeChore, TypeMessage, TypeMergeRequest, TypeMolecule, TypeGate, TypeAgent, TypeRole:
+	case TypeBug, TypeFeature, TypeTask, TypeEpic, TypeChore, TypeMessage, TypeMergeRequest, TypeMolecule, TypeGate, TypeAgent, TypeRole, TypeConvoy:
 		return true
 	}
 	return false