feat: AI marker system prompt fixes, TaskLane activation, and FlowPilot updates

- Fix system prompt to ensure [QUESTIONS]/[ACTIONS] markers in AI responses
- Add format reminder injection to user messages for marker compliance
- Wire TaskLane activation in prefill and resume paths
- Add ActionCardGroup component for structured question/action rendering
- Update FlowPilot session and step card components
- Update ai-session schemas and types for marker data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/ai_session.py

@@ -250,10 +250,40 @@ class ChatMessageRequest(BaseModel):
     upload_ids: list[UUID] = Field(default_factory=list, max_length=10)
 
 
+class ForkBranchInfo(BaseModel):
+    """Branch info returned when a fork is created."""
+    branch_id: str
+    label: str
+
+
+class ForkMetadata(BaseModel):
+    """Metadata returned when the AI suggests a diagnostic fork."""
+    fork_point_id: str
+    fork_reason: str
+    branches: list[ForkBranchInfo]
+    active_branch_id: str
+
+
+class ActionItem(BaseModel):
+    """A single action item for the engineer."""
+    label: str
+    command: str | None = None
+    description: str = ""
+
+
+class QuestionItem(BaseModel):
+    """A question the AI needs answered by the engineer."""
+    text: str
+    context: str = ""
+
+
 class ChatMessageResponse(BaseModel):
     """AI response to a chat message."""
     content: str
     suggested_flows: list[dict[str, Any]] = []
+    fork: ForkMetadata | None = None
+    actions: list[ActionItem] | None = None
+    questions: list[QuestionItem] | None = None
 
 
 class AISessionSearchResult(BaseModel):