fix(perception+brain): story view detection + autonomous prompt

Two root causes for 'scroll on story' bug:

1. ScreenIdentity had ZERO structural markers for story views.
   reel_viewer_media_layout, reel_viewer_header, reel_viewer_progress_bar
   and content-desc 'Like Story'/'Send story' now → STORY_VIEW.

2. Brain prompt was prescriptive ('you MUST scroll down'), overriding
   the LLM's intelligence. Rewritten to give context about screen types
   and let the AI reason autonomously about which action makes sense.

Philosophy: AI decides navigation, we provide correct perception data.
No hardcoded 'if story → press back' escape hatch.

4 new perception tests (all green), 0 regressions.

tests/e2e/test_engine_perception.py

@@ -363,3 +363,4 @@ class TestStoryViewDetection:
         assert len(tab_actions) == 0, (
             f"Story view should have NO tab navigation, but found: {tab_actions}"
         )
+