Implement cycle detection and prevention improvements

- Add diagnostic warnings when cycles detected after dep add (bd-309)
- Add semantic validation for parent-child dependency direction (bd-308)
- Document cycle handling behavior in code, README, and DESIGN (bd-310)

Changes:
- cmd/bd/dep.go: Add DetectCycles() call and warning after dep add
- internal/storage/sqlite/dependencies.go: Add parent-child direction validation and comprehensive cycle prevention comments
- internal/storage/sqlite/dependencies_test.go: Add TestParentChildValidation
- README.md: Add dependency types and cycle prevention section with examples
- DESIGN.md: Add detailed cycle prevention design rationale and trade-offs

cmd/bd/dep.go

@@ -31,26 +31,51 @@ var depAddCmd = &cobra.Command{
 
 		ctx := context.Background()
 		if err := store.AddDependency(ctx, dep, actor); err != nil {
-			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
-			os.Exit(1)
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
 		}
 
 		// Schedule auto-flush
 		markDirtyAndScheduleFlush()
 
-		if jsonOutput {
-			outputJSON(map[string]interface{}{
-				"status":       "added",
-				"issue_id":     args[0],
-				"depends_on_id": args[1],
-				"type":         depType,
-			})
-			return
+		// Check for cycles after adding dependency
+		cycles, err := store.DetectCycles(ctx)
+		if err != nil {
+		fmt.Fprintf(os.Stderr, "Warning: Failed to check for cycles: %v\n", err)
+		} else if len(cycles) > 0 {
+		yellow := color.New(color.FgYellow).SprintFunc()
+		fmt.Fprintf(os.Stderr, "\n%s Warning: Dependency cycle detected!\n", yellow("⚠"))
+		fmt.Fprintf(os.Stderr, "This can hide issues from the ready work list and cause confusion.\n\n")
+		 fmt.Fprintf(os.Stderr, "Cycle path:\n")
+		for _, cycle := range cycles {
+		  for j, issue := range cycle {
+		   if j == 0 {
+		   fmt.Fprintf(os.Stderr, "  %s", issue.ID)
+				} else {
+					fmt.Fprintf(os.Stderr, " → %s", issue.ID)
+				}
+			}
+			if len(cycle) > 0 {
+				fmt.Fprintf(os.Stderr, " → %s", cycle[0].ID)
+			}
+			fmt.Fprintf(os.Stderr, "\n")
 		}
+		fmt.Fprintf(os.Stderr, "\nRun 'bd dep cycles' for detailed analysis.\n\n")
+	}
 
-		green := color.New(color.FgGreen).SprintFunc()
-		fmt.Printf("%s Added dependency: %s depends on %s (%s)\n",
-			green("✓"), args[0], args[1], depType)
+	if jsonOutput {
+		outputJSON(map[string]interface{}{
+			"status":       "added",
+			"issue_id":     args[0],
+			"depends_on_id": args[1],
+			"type":         depType,
+		})
+		return
+	}
+
+	green := color.New(color.FgGreen).SprintFunc()
+	fmt.Printf("%s Added dependency: %s depends on %s (%s)\n",
+		green("✓"), args[0], args[1], depType)
 	},
 }