fix(docs,test): correct if/undefined semantics doc and drop redundant del

The task-level `if:` condition treats an undefined name as falsy and skips the
task (the runner catches UndefinedError), so correct GRAMMAR.md which claimed it
raises; also clarify that in-prompt templating still raises on undefined, unlike
`if`. Remove an unnecessary `del` in a stream test helper flagged by code scanning.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: anticomputer

doc/GRAMMAR.md

@@ -223,8 +223,9 @@ Notes:
 - The expression may be written bare (`globals.mode == 'deep'`) or wrapped in
   `{{ ... }}`. Standard truthiness applies (empty list/string/`0`/`false` are
   falsy).
-- The condition is evaluated with `StrictUndefined`, so referencing a name that
-  does not exist raises rather than silently skipping. Guard optional data with
+- Referencing a name that does not exist (for example an output from a task that
+  has not run) is treated as falsy, so the task is skipped rather than failing,
+  matching GitHub Actions semantics. To be explicit you can still guard with
   `is defined`, e.g. `if: "outputs.audit is defined and outputs.audit.findings"`.
 - `if` composes with everything else: a skipped task does not run its agents,
   fan out over `models`, or capture outputs.
@@ -248,8 +249,10 @@ directly inside a `user_prompt`:
         {% endfor %}
 ```
 
-As with `if`, undefined variables raise (to catch typos). Use `is defined` or
-the `default` filter for optional data: `{{ globals.note | default('') }}`.
+Unlike the task-level `if` condition (which treats undefined names as falsy and
+skips the task), undefined variables inside a prompt raise, to catch typos. Use
+`is defined` or the `default` filter for optional data:
+`{{ globals.note | default('') }}`.
 
 ### Running templated tasks in a loop
 

tests/test_stream.py

@@ -115,8 +115,8 @@ async def _fake_render(text: str, **_kw: Any) -> None:
 
 
 def test_drive_forwards_token_usage(monkeypatch):
-    async def _fake_render(text: str, **_kw: Any) -> None:
-        del text
+    async def _fake_render(*_a: Any, **_kw: Any) -> None:
+        return None
 
     monkeypatch.setattr("seclab_taskflow_agent._stream.render_model_output", _fake_render)