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/Taskfile.yml

@@ -8,11 +8,13 @@ vars:
   VAR: value_from_a
   UNIQUE_A: only_in_a
 
+includes:
+  nested: ./nested
+
 tasks:
   print:
     desc: Print vars from include A
     cmds:
-      - echo "A:VAR={{.VAR}}"
       - echo "A:UNIQUE_A={{.UNIQUE_A}}"
       - echo "A:ROOT_VAR={{.ROOT_VAR}}"
 

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}}"

testdata/scoped_taskfiles/inc_b/Taskfile.yml

@@ -12,7 +12,6 @@ tasks:
   print:
     desc: Print vars from include B
     cmds:
-      - echo "B:VAR={{.VAR}}"
       - echo "B:UNIQUE_B={{.UNIQUE_B}}"
       - echo "B:ROOT_VAR={{.ROOT_VAR}}"
 

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-legacy-default.golden

@@ -1,6 +1,4 @@
-A:VAR=value_from_b
 A:UNIQUE_A=only_in_a
 A:ROOT_VAR=from_root
-B:VAR=value_from_b
 B:UNIQUE_B=only_in_b
 B:ROOT_VAR=from_root

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-default.golden

@@ -1,6 +1,4 @@
-A:VAR=value_from_a
 A:UNIQUE_A=only_in_a
 A:ROOT_VAR=from_root
-B:VAR=value_from_b
 B:UNIQUE_B=only_in_b
 B:ROOT_VAR=from_root

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-inheritance-a.golden

@@ -1,3 +1,2 @@
-A:VAR=value_from_a
 A:UNIQUE_A=only_in_a
 A:ROOT_VAR=from_root

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-isolation-b.golden

@@ -1,3 +1,2 @@
-B:VAR=value_from_b
 B:UNIQUE_B=only_in_b
 B:ROOT_VAR=from_root

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-nested.golden

@@ -0,0 +1,5 @@
+NESTED:ROOT_VAR=from_root
+NESTED:UNIQUE_A=only_in_a
+NESTED:NESTED_VAR=from_nested
+NESTED:NESTED_ENV=env_from_nested
+NESTED:ROOT_ENV=env_from_root