Let the agent copy nodes, also into another graph (#434)

The agent built every part again that already stood in a graph,
node by node, while the editor has Ctrl+C and Ctrl+V for it. The new
command `copy_nodes` in the area graphs copies nodes into the same
graph or another one, in one undo step with the rest of the call.

- The links between the copied nodes come along, and so do groups
  and comments that are only about copied nodes. A node belongs to a
  group by its middle, as when it is dragged. `boxesAbout` in
  annotationFollow.ts decides this.
- `made` names each copy by the id of its original, so later steps
  link to it with `$copy.<id>`. `$copy` alone is the first one.
- Without x and y the copies are arranged at the end of the call like
  new nodes. With them they keep their layout, top left corner there.
- Nodes the target graph does not take are refused by name, not left
  out as Ctrl+V does. The rule is the graph tab's: `fitsGraph` and
  `fittingPart` moved from clipboard.ts to paletteEntries.ts.
- A copied Variables node takes the variables of its behavior along
  into the target behavior (`CarryVariablesCommand`), with default,
  value, Keep and Private. A variable of that name of another kind
  there is refused.
- The relay's instructions name `copy_nodes`.

Found on the way:
- Groups made by the agent can take in a node of another group once
  the nodes are arranged. lint_graph does not count that.
- Undo after a paste leaves an empty `groups: []`, as
  AddAnnotationCommand does. It is the same as no list.

Tests:
- agent-copy (new, 3 tests) with the agent, relay, clipboard and
  palette tests: 118/118.
- Counter-probes: 6 bendings, all compile, all red.
- e2e 71 (atx): ALL PASSED, 26 checks.
- Gate (atx): CHECK EXIT 0, 327 files, 5024 tests.
- Live in the MCP Test Project: 5 nodes of Arrow keys walk into a new
  behavior, 6 links, group, comment and speed (300) along, lint_graph
  0 everywhere, taken back with undo.

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

src/core/commands/variableCommands.ts

@@ -65,6 +65,48 @@ export class AddVariableCommand implements Command {
   }
 }
 
+/**
+ * Declares variables of one behavior in another as they are declared there (#434): default, Keep
+ * and Private included, for copied nodes that name them. A key the target has already is left alone.
+ */
+export class CarryVariablesCommand implements Command {
+  readonly label = 'Carry variables';
+  private added: string[] = [];
+
+  constructor(
+    private readonly assetId: string,
+    private readonly fromId: string,
+    private readonly keys: readonly string[],
+  ) {}
+
+  execute(project: Project): void {
+    const from = requireBehavior(project, this.fromId);
+    const to = requireBehavior(project, this.assetId);
+    this.added = [];
+    for (const key of this.keys) {
+      const def = from.variables.find((v) => v.key === key);
+      if (!def) throw new CommandError(`${from.name} has no variable "${key}"`);
+      if (to.variables.some((v) => v.key === key)) continue;
+      to.variables.push(structuredClone(def));
+      to.props[key] = structuredClone(from.props[key] ?? def.default);
+      if (isKeptVariable(from, key)) setKeptVariable(to, key, true);
+      if (isPrivateVariable(from, key)) setPrivateVariable(to, key, true);
+      this.added.push(key);
+    }
+  }
+
+  undo(project: Project): void {
+    const to = requireBehavior(project, this.assetId);
+    const added = new Set(this.added);
+    to.variables = to.variables.filter((v) => !added.has(v.key));
+    for (const key of added) {
+      delete to.props[key];
+      setKeptVariable(to, key, false);
+      setPrivateVariable(to, key, false);
+    }
+  }
+}
+
 /** One placed object's own value for a variable, remembered so that undo can put it back. */
 interface TakenOverride {
   holderId: string;