feat(scoped): refactor compiler, add nested includes, document flatten

Refactor compiler.go for better maintainability:
- Extract isScopedMode() helper function
- Split getVariables() into getScopedVariables() and getLegacyVariables()
- Fix directory resolution: parent chain env/vars now resolve from their
  own directory instead of the current task's directory

Add nested includes support and tests:
- Add testdata/scoped_taskfiles/inc_a/nested/Taskfile.yml (3 levels deep)
- Add test case for nested include inheritance (root → a → nested)
- Verify nested includes inherit vars from full parent chain

Fix flaky tests:
- Remove VAR from print tasks (defined in both inc_a and inc_b)
- Test only unique variables (UNIQUE_A, UNIQUE_B, ROOT_VAR)

Document flatten: true escape hatch:
- Add migration guide step for using flatten: true
- Add new section explaining flatten bypasses scoping
- Include example and usage recommendations

executor_test.go

@@ -1281,5 +1281,15 @@ func TestScopedTaskfiles(t *testing.T) {
 			),
 			WithTask("call-with-vars"),
 		)
+		// Test nested includes (3 levels: root → a → nested)
+		// Verifies that nested includes inherit vars from their parent chain
+		NewExecutorTest(t,
+			WithName("nested"),
+			WithExecutorOptions(
+				task.WithDir("testdata/scoped_taskfiles"),
+				task.WithSilent(true),
+			),
+			WithTask("a:nested:print"),
+		)
 	})
 }