feat(beads): add cycle detection for molecule dependencies

Implement DFS-based cycle detection in ValidateMolecule to catch
circular dependencies in molecule step graphs. The algorithm uses
three-color marking (unvisited/visiting/visited) to detect back
edges that indicate cycles.

When a cycle is detected, the error message shows the cycle path
(e.g., "a -> b -> c -> a") for easy debugging.

Add 4 new tests:
- SimpleCycle: A -> B -> A
- LongerCycle: A -> B -> C -> A
- DiamondNoCycle: ensures valid diamond patterns pass
- CycleInSubgraph: cycle not involving root node

Closes gt-ai1z.

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

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

internal/beads/molecule_test.go

@@ -2,6 +2,7 @@ package beads
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -489,3 +490,108 @@ Has content.`
 		t.Errorf("step[1].Instructions = %q", steps[1].Instructions)
 	}
 }
+
+func TestValidateMolecule_SimpleCycle(t *testing.T) {
+	// A -> B -> A (simple 2-node cycle)
+	mol := &Issue{
+		ID:   "mol-xyz",
+		Type: "molecule",
+		Description: `## Step: a
+First step.
+Needs: b
+
+## Step: b
+Second step.
+Needs: a`,
+	}
+
+	err := ValidateMolecule(mol)
+	if err == nil {
+		t.Error("ValidateMolecule() = nil, want error for cycle")
+	}
+	if err != nil && !strings.Contains(err.Error(), "cycle") {
+		t.Errorf("error %q should mention 'cycle'", err.Error())
+	}
+}
+
+func TestValidateMolecule_LongerCycle(t *testing.T) {
+	// A -> B -> C -> A (3-node cycle)
+	mol := &Issue{
+		ID:   "mol-xyz",
+		Type: "molecule",
+		Description: `## Step: a
+First step.
+Needs: c
+
+## Step: b
+Second step.
+Needs: a
+
+## Step: c
+Third step.
+Needs: b`,
+	}
+
+	err := ValidateMolecule(mol)
+	if err == nil {
+		t.Error("ValidateMolecule() = nil, want error for cycle")
+	}
+	if err != nil && !strings.Contains(err.Error(), "cycle") {
+		t.Errorf("error %q should mention 'cycle'", err.Error())
+	}
+}
+
+func TestValidateMolecule_DiamondNoCycle(t *testing.T) {
+	// Diamond pattern: A -> B, A -> C, B -> D, C -> D
+	// This has no cycle, should pass
+	mol := &Issue{
+		ID:   "mol-xyz",
+		Type: "molecule",
+		Description: `## Step: a
+Root step.
+
+## Step: b
+Branch 1.
+Needs: a
+
+## Step: c
+Branch 2.
+Needs: a
+
+## Step: d
+Merge point.
+Needs: b, c`,
+	}
+
+	err := ValidateMolecule(mol)
+	if err != nil {
+		t.Errorf("ValidateMolecule() = %v, want nil (diamond has no cycle)", err)
+	}
+}
+
+func TestValidateMolecule_CycleInSubgraph(t *testing.T) {
+	// Root -> A, A -> B -> C -> A (cycle not involving root)
+	mol := &Issue{
+		ID:   "mol-xyz",
+		Type: "molecule",
+		Description: `## Step: root
+Starting point.
+
+## Step: a
+First in cycle.
+Needs: root, c
+
+## Step: b
+Second in cycle.
+Needs: a
+
+## Step: c
+Third in cycle.
+Needs: b`,
+	}
+
+	err := ValidateMolecule(mol)
+	if err == nil {
+		t.Error("ValidateMolecule() = nil, want error for cycle in subgraph")
+	}
+}