feat(experiments): rename SCOPED_INCLUDES to SCOPED_TASKFILES and add env namespace

Rename the experiment from SCOPED_INCLUDES to SCOPED_TASKFILES to better
reflect its expanded scope. This experiment now provides:

1. Variable scoping (existing): includes see only their own vars + parent vars
2. Environment namespace (new): env vars accessible via {{.env.XXX}}

With TASK_X_SCOPED_TASKFILES=1:
- {{.VAR}} accesses vars only (scoped per include)
- {{.env.VAR}} accesses env (OS + Taskfile env:, inherited)
- {{.TASK}} and other special vars remain at root level

This is a breaking change for the experimental feature:
- {{.PATH}} no longer works, use {{.env.PATH}} instead
- Env vars are no longer at root level in templates

testdata/scoped_includes/Taskfile.yml

@@ -1,20 +0,0 @@
-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}}"

testdata/scoped_includes/inc_a/Taskfile.yml

@@ -1,18 +0,0 @@
-version: "3"
-
-vars:
-  VAR: value_from_a
-  UNIQUE_A: only_in_a
-
-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}}"
-
-  try-access-b:
-    desc: Try to access B's unique var (should fail in scoped mode)
-    cmds:
-      - echo "A:UNIQUE_B={{.UNIQUE_B}}"

testdata/scoped_includes/inc_b/Taskfile.yml

@@ -1,13 +0,0 @@
-version: "3"
-
-vars:
-  VAR: value_from_b
-  UNIQUE_B: only_in_b
-
-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/Taskfile.yml

@@ -0,0 +1,38 @@
+version: "3"
+
+env:
+  ROOT_ENV: env_from_root
+  SHARED_ENV: shared_from_root
+
+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}}"
+
+  print-env:
+    desc: Print env vars using {{.env.XXX}} syntax
+    cmds:
+      - echo "ROOT_ENV={{.env.ROOT_ENV}}"
+      - echo "SHARED_ENV={{.env.SHARED_ENV}}"
+      - echo "PATH_EXISTS={{if .env.PATH}}yes{{else}}no{{end}}"
+
+  test-env-separation:
+    desc: Test that env is NOT at root level in scoped mode
+    cmds:
+      # In scoped mode, {{.ROOT_ENV}} should be empty (env not at root)
+      # In legacy mode, {{.ROOT_ENV}} would have the value
+      - echo "ROOT_ENV_AT_ROOT={{.ROOT_ENV}}"

testdata/scoped_taskfiles/inc_a/Taskfile.yml

@@ -0,0 +1,36 @@
+version: "3"
+
+env:
+  INC_A_ENV: env_from_a
+  SHARED_ENV: shared_from_a
+
+vars:
+  VAR: value_from_a
+  UNIQUE_A: only_in_a
+
+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}}"
+
+  try-access-b:
+    desc: Try to access B's unique var (should fail in scoped mode)
+    cmds:
+      - echo "A:UNIQUE_B={{.UNIQUE_B}}"
+
+  print-env:
+    desc: Print env vars from include A
+    cmds:
+      - echo "A:INC_A_ENV={{.env.INC_A_ENV}}"
+      - echo "A:ROOT_ENV={{.env.ROOT_ENV}}"
+      - echo "A:SHARED_ENV={{.env.SHARED_ENV}}"
+
+  test-env-in-var:
+    desc: Test using env in a var template
+    vars:
+      COMPOSED: "env={{.env.ROOT_ENV}}"
+    cmds:
+      - echo "{{.COMPOSED}}"

testdata/scoped_taskfiles/inc_b/Taskfile.yml

@@ -0,0 +1,24 @@
+version: "3"
+
+env:
+  INC_B_ENV: env_from_b
+  SHARED_ENV: shared_from_b
+
+vars:
+  VAR: value_from_b
+  UNIQUE_B: only_in_b
+
+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}}"
+
+  print-env:
+    desc: Print env vars from include B
+    cmds:
+      - echo "B:INC_B_ENV={{.env.INC_B_ENV}}"
+      - echo "B:ROOT_ENV={{.env.ROOT_ENV}}"
+      - echo "B:SHARED_ENV={{.env.SHARED_ENV}}"

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-env-namespace.golden

@@ -0,0 +1,3 @@
+ROOT_ENV=env_from_root
+SHARED_ENV=shared_from_root
+PATH_EXISTS=yes

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-env-separation.golden

@@ -0,0 +1 @@
+ROOT_ENV_AT_ROOT=

testdata/scoped_taskfiles/testdata/TestScopedTaskfiles-scoped-include-env.golden

@@ -0,0 +1,3 @@
+A:INC_A_ENV=env_from_a
+A:ROOT_ENV=env_from_root
+A:SHARED_ENV=shared_from_a