Diffs name their chain; history shows the ruleset's diff

The ruleset's diff names the chain each hunk is in on its @@ line, as
git names the function. A history entry shows what it did to the
ruleset (web page, hc history diff, GET /history/{id}/diff), rebuilt
from the current configuration by stepping back through the later
entries. A changed setting is listed by its own fields
(builtins.drop_invalid) instead of the whole object.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/hc/admin.go

@@ -66,6 +66,30 @@ func historyCmd() *cobra.Command {
 			})
 		},
 	})
+	cmd.AddCommand(&cobra.Command{
+		Use: "diff ID", Short: "Show what a change did to the ruleset", Args: cobra.ExactArgs(1),
+		Long: `Shows the ruleset's diff for a change: the configuration before and after
+it, rendered by this version of Housecarl.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newClient()
+			if err != nil {
+				return err
+			}
+			var d struct {
+				Diff string `json:"diff"`
+			}
+			if err := c.Get(ctx(), "/history/"+args[0]+"/diff", nil, &d); err != nil {
+				return err
+			}
+			return output(d, func(w io.Writer) {
+				if d.Diff == "" {
+					fmt.Fprintln(w, "the ruleset didn't change")
+					return
+				}
+				printDiff(w, d.Diff)
+			})
+		},
+	})
 	cmd.AddCommand(&cobra.Command{
 		Use:   "undo ID",
 		Short: "Undo a change",
@@ -88,11 +112,13 @@ change touched has changed since; undo the later changes first.`,
 }
 
 // diffJSON lists the fields that differ between two versions of an
-// object: "name: office -> lab".
+// object: "name: office -> lab". An object within (settings' builtins)
+// is compared field by field: "builtins.drop_invalid: false -> true".
 func diffJSON(before, after json.RawMessage) []string {
 	var a, b map[string]any
 	json.Unmarshal(before, &a)
 	json.Unmarshal(after, &b)
+	a, b = flatten(a, "", map[string]any{}), flatten(b, "", map[string]any{})
 	keys := map[string]bool{}
 	for k := range a {
 		keys[k] = true
@@ -132,6 +158,19 @@ func diffJSON(before, after json.RawMessage) []string {
 	return out
 }
 
+// flatten lifts the fields of objects within m to dotted keys; lists
+// stay whole.
+func flatten(m map[string]any, prefix string, out map[string]any) map[string]any {
+	for k, v := range m {
+		if o, ok := v.(map[string]any); ok && len(o) > 0 {
+			flatten(o, prefix+k+".", out)
+		} else {
+			out[prefix+k] = v
+		}
+	}
+	return out
+}
+
 func whoamiCmd() *cobra.Command {
 	return &cobra.Command{
 		Use: "whoami", Short: "Show who you are to the server", Args: cobra.NoArgs,