feat(brain): add GrowthBrain.select_task() + kill abstract goals config

- GrowthBrain.select_task() uses weighted random from concrete Task objects
- Removed self.goals from Config (no longer reads goals: from config.yml)
- Mission + plugins are now the SSOT for bot behavior

The bot no longer receives abstract strings like 'Nurture my community' that
the LLM Brain can't operationalize. Instead, the GoalDecomposer generates
Task(browse_feed, HomeFeed, budget=7) which routes to concrete feed loops.

18/18 TDD tests passing.

tests/tdd/test_goal_decomposer.py

@@ -210,3 +210,110 @@ class TestGoalDecomposerGeneratesTasks:
                 f"Task target_screen '{task.target_screen}' is not in ScreenTopology! "
                 f"The bot cannot navigate there."
             )
+
+
+class TestGoalDecomposerFromConfig:
+    """Tests that GoalDecomposer correctly derives tasks from config-like dicts.
+    Validates that the legacy `goals:` config is not needed."""
+
+    def test_decomposer_produces_tasks_without_goals(self):
+        """Tasks come from plugins + actions, NOT from goals list."""
+        from GramAddict.core.goal_decomposer import GoalDecomposer
+
+        plugins = {"likes": {"percentage": 100}}
+        actions = {"feed": "5-10"}
+        mission = {"strategy": "aggressive_growth"}
+
+        decomposer = GoalDecomposer(plugins=plugins, actions=actions, mission=mission)
+        tasks = decomposer.generate_tasks()
+
+        assert len(tasks) > 0, "Tasks must come from plugins, not goals"
+
+    def test_decomposer_accepts_empty_mission(self):
+        """If no mission is provided, default to aggressive_growth."""
+        from GramAddict.core.goal_decomposer import GoalDecomposer
+
+        plugins = {"likes": {"percentage": 100}}
+        actions = {"feed": "5-10"}
+
+        decomposer = GoalDecomposer(plugins=plugins, actions=actions, mission={})
+        tasks = decomposer.generate_tasks()
+
+        assert len(tasks) > 0, "Empty mission must fall back to aggressive_growth"
+
+
+class TestGrowthBrainTaskSelection:
+    """Tests that GrowthBrain.select_task() picks from concrete Task objects."""
+
+    def test_select_task_returns_task_object(self):
+        from GramAddict.core.goal_decomposer import GoalDecomposer, Task
+        from GramAddict.core.growth_brain import GrowthBrain
+        from GramAddict.core.dopamine_engine import DopamineEngine
+
+        plugins = {"likes": {"percentage": 100}, "story_view": {"percentage": 80}}
+        actions = {"feed": "5-10", "explore": "5-10"}
+        mission = {"strategy": "aggressive_growth"}
+
+        decomposer = GoalDecomposer(plugins=plugins, actions=actions, mission=mission)
+        tasks = decomposer.generate_tasks()
+        dopamine = DopamineEngine()
+
+        brain = GrowthBrain(username="test", persona_interests=[])
+        selected = brain.select_task(dopamine, tasks)
+
+        assert isinstance(selected, Task), f"Expected Task, got {type(selected)}"
+        assert selected in tasks
+
+    def test_select_task_returns_none_on_empty_tasks(self):
+        from GramAddict.core.growth_brain import GrowthBrain
+        from GramAddict.core.dopamine_engine import DopamineEngine
+
+        dopamine = DopamineEngine()
+        brain = GrowthBrain(username="test", persona_interests=[])
+        selected = brain.select_task(dopamine, [])
+
+        assert selected is None, "Empty task list must return None"
+
+    def test_select_task_returns_none_on_high_boredom(self):
+        from GramAddict.core.goal_decomposer import GoalDecomposer
+        from GramAddict.core.growth_brain import GrowthBrain
+        from GramAddict.core.dopamine_engine import DopamineEngine
+
+        plugins = {"likes": {"percentage": 100}}
+        actions = {"feed": "5-10"}
+        mission = {"strategy": "aggressive_growth"}
+
+        decomposer = GoalDecomposer(plugins=plugins, actions=actions, mission=mission)
+        tasks = decomposer.generate_tasks()
+        dopamine = DopamineEngine()
+        dopamine.boredom = 95.0
+
+        brain = GrowthBrain(username="test", persona_interests=[])
+        selected = brain.select_task(dopamine, tasks)
+
+        assert selected is None, "High boredom must return None (= ShiftContext signal)"
+
+    def test_select_task_uses_weights(self):
+        """Over many selections, higher-weighted tasks should appear more often."""
+        from GramAddict.core.goal_decomposer import GoalDecomposer, Task
+        from GramAddict.core.growth_brain import GrowthBrain
+        from GramAddict.core.dopamine_engine import DopamineEngine
+
+        plugins = {"likes": {"percentage": 100}, "story_view": {"percentage": 80}}
+        actions = {"feed": "5-10", "explore": "5-10"}
+        mission = {"strategy": "aggressive_growth"}
+
+        decomposer = GoalDecomposer(plugins=plugins, actions=actions, mission=mission)
+        tasks = decomposer.generate_tasks()
+        dopamine = DopamineEngine()
+        brain = GrowthBrain(username="test", persona_interests=[])
+
+        # Run 100 selections and count
+        counts: dict = {}
+        for _ in range(100):
+            selected = brain.select_task(dopamine, tasks)
+            if selected:
+                counts[selected.target_screen] = counts.get(selected.target_screen, 0) + 1
+
+        # With aggressive_growth, ExploreFeed (weight 0.45) should dominate
+        assert len(counts) > 1, "Selection must pick from multiple screens"