test(scoped-includes): add tests for variable isolation

Tests verify:
- Legacy mode: vars merged globally (A sees B's VAR, can access UNIQUE_B)
- Scoped mode: vars isolated (A sees own VAR, cannot access UNIQUE_B)
- Inheritance: includes can still access root vars (ROOT_VAR)

Test structure:
- testdata/scoped_includes/ with main Taskfile and two includes
- inc_a and inc_b both define VAR with different values
- Cross-include test shows A trying to access B's UNIQUE_B

testdata/scoped_includes/Taskfile.yml

@@ -0,0 +1,20 @@
+version: "3"
+
+vars:
+  ROOT_VAR: from_root
+
+includes:
+  a: ./inc_a
+  b: ./inc_b
+
+tasks:
+  default:
+    desc: Test scoped includes - vars should be isolated
+    cmds:
+      - task: a:print
+      - task: b:print
+
+  print-root-var:
+    desc: Print ROOT_VAR from root
+    cmds:
+      - echo "ROOT_VAR={{.ROOT_VAR}}"