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

testdata/scoped_taskfiles/inc_a/nested/Taskfile.yml

@@ -0,0 +1,22 @@
+version: "3"
+
+env:
+  NESTED_ENV: env_from_nested
+
+vars:
+  NESTED_VAR: from_nested
+
+tasks:
+  print:
+    desc: Print vars from nested include (3 levels deep)
+    cmds:
+      - echo "NESTED:ROOT_VAR={{.ROOT_VAR}}"
+      - echo "NESTED:UNIQUE_A={{.UNIQUE_A}}"
+      - echo "NESTED:NESTED_VAR={{.NESTED_VAR}}"
+      - echo "NESTED:NESTED_ENV={{.env.NESTED_ENV}}"
+      - echo "NESTED:ROOT_ENV={{.env.ROOT_ENV}}"
+
+  try-access-b:
+    desc: Try to access B's unique var (should fail - sibling isolation)
+    cmds:
+      - echo "NESTED:UNIQUE_B={{.UNIQUE_B}}"