Add delegation relationship type to beads (gt-6r18e.5)

Add Delegation and DelegationTerms structs for tracking work delegation
between work units. This enables the HOP pattern of work flowing down
and credit cascading up.

New types:
- Delegation: Links parent and child work units with delegated_by/to
- DelegationTerms: Optional terms including portion, deadline, credit_share

New functions:
- AddDelegation: Create a delegation relationship
- RemoveDelegation: Remove a delegation relationship
- GetDelegation: Retrieve delegation info for a child work unit
- ListDelegationsFrom: List all delegations from a parent

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

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

internal/beads/beads_test.go

@@ -1,6 +1,7 @@
 package beads
 
 import (
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"strings"
@@ -1377,3 +1378,102 @@ func TestRoleBeadID(t *testing.T) {
 		})
 	}
 }
+
+// TestDelegationStruct tests the Delegation struct serialization.
+func TestDelegationStruct(t *testing.T) {
+	tests := []struct {
+		name       string
+		delegation Delegation
+		wantJSON   string
+	}{
+		{
+			name: "full delegation",
+			delegation: Delegation{
+				Parent:      "hop://accenture.com/eng/proj-123/task-a",
+				Child:       "hop://alice@example.com/main-town/gastown/gt-xyz",
+				DelegatedBy: "hop://accenture.com",
+				DelegatedTo: "hop://alice@example.com",
+				Terms: &DelegationTerms{
+					Portion:     "backend-api",
+					Deadline:    "2025-06-01",
+					CreditShare: 80,
+				},
+				CreatedAt: "2025-01-15T10:00:00Z",
+			},
+			wantJSON: `{"parent":"hop://accenture.com/eng/proj-123/task-a","child":"hop://alice@example.com/main-town/gastown/gt-xyz","delegated_by":"hop://accenture.com","delegated_to":"hop://alice@example.com","terms":{"portion":"backend-api","deadline":"2025-06-01","credit_share":80},"created_at":"2025-01-15T10:00:00Z"}`,
+		},
+		{
+			name: "minimal delegation",
+			delegation: Delegation{
+				Parent:      "gt-abc",
+				Child:       "gt-xyz",
+				DelegatedBy: "steve",
+				DelegatedTo: "alice",
+			},
+			wantJSON: `{"parent":"gt-abc","child":"gt-xyz","delegated_by":"steve","delegated_to":"alice"}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := json.Marshal(tt.delegation)
+			if err != nil {
+				t.Fatalf("json.Marshal failed: %v", err)
+			}
+			if string(got) != tt.wantJSON {
+				t.Errorf("json.Marshal = %s, want %s", string(got), tt.wantJSON)
+			}
+
+			// Test round-trip
+			var parsed Delegation
+			if err := json.Unmarshal(got, &parsed); err != nil {
+				t.Fatalf("json.Unmarshal failed: %v", err)
+			}
+			if parsed.Parent != tt.delegation.Parent {
+				t.Errorf("parsed.Parent = %s, want %s", parsed.Parent, tt.delegation.Parent)
+			}
+			if parsed.Child != tt.delegation.Child {
+				t.Errorf("parsed.Child = %s, want %s", parsed.Child, tt.delegation.Child)
+			}
+			if parsed.DelegatedBy != tt.delegation.DelegatedBy {
+				t.Errorf("parsed.DelegatedBy = %s, want %s", parsed.DelegatedBy, tt.delegation.DelegatedBy)
+			}
+			if parsed.DelegatedTo != tt.delegation.DelegatedTo {
+				t.Errorf("parsed.DelegatedTo = %s, want %s", parsed.DelegatedTo, tt.delegation.DelegatedTo)
+			}
+		})
+	}
+}
+
+// TestDelegationTerms tests the DelegationTerms struct.
+func TestDelegationTerms(t *testing.T) {
+	terms := &DelegationTerms{
+		Portion:            "frontend",
+		Deadline:           "2025-03-15",
+		AcceptanceCriteria: "All tests passing, code reviewed",
+		CreditShare:        70,
+	}
+
+	got, err := json.Marshal(terms)
+	if err != nil {
+		t.Fatalf("json.Marshal failed: %v", err)
+	}
+
+	var parsed DelegationTerms
+	if err := json.Unmarshal(got, &parsed); err != nil {
+		t.Fatalf("json.Unmarshal failed: %v", err)
+	}
+
+	if parsed.Portion != terms.Portion {
+		t.Errorf("parsed.Portion = %s, want %s", parsed.Portion, terms.Portion)
+	}
+	if parsed.Deadline != terms.Deadline {
+		t.Errorf("parsed.Deadline = %s, want %s", parsed.Deadline, terms.Deadline)
+	}
+	if parsed.AcceptanceCriteria != terms.AcceptanceCriteria {
+		t.Errorf("parsed.AcceptanceCriteria = %s, want %s", parsed.AcceptanceCriteria, terms.AcceptanceCriteria)
+	}
+	if parsed.CreditShare != terms.CreditShare {
+		t.Errorf("parsed.CreditShare = %d, want %d", parsed.CreditShare, terms.CreditShare)
+	}
+}