feat: add cockpit triage backend foundation (Phase 1)

- Migration 071: add client_name, asset_name, issue_category,
  triage_hypothesis, evidence_items columns to ai_sessions
- TriageUpdate schema for AI-inferred header updates in chat responses
- QuestionItem.options field for quick-reply buttons
- PATCH /ai-sessions/{id}/triage endpoint for manual header edits
- POST /ai-sessions/{id}/handoff-draft streaming endpoint for conclude modal
- Structured handoff fields (root_cause, steps_taken, recommendations)
  on resolve/escalate requests, passed through to ResolutionOutputGenerator
- Triage fields exposed in AISessionDetail response for session resume

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

backend/app/schemas/ai_session.py

@@ -102,12 +102,20 @@ class ResolveSessionRequest(BaseModel):
     resolution_action: str | None = None
     session_rating: int | None = Field(None, ge=1, le=5)
     session_feedback: str | None = None
+    # Structured handoff fields (from cockpit conclude modal)
+    root_cause: str | None = None
+    steps_taken: list[str] | None = None
+    recommendations: str | None = None
 
 
 class EscalateSessionRequest(BaseModel):
     """Escalate a session to another engineer."""
     escalation_reason: str = Field(..., min_length=5, max_length=2000)
     escalated_to_id: UUID | None = None
+    # Structured handoff fields (from cockpit conclude modal)
+    root_cause: str | None = None
+    steps_taken: list[str] | None = None
+    recommendations: str | None = None
 
 
 class DocumentationStep(BaseModel):
@@ -231,6 +239,12 @@ class AISessionDetail(AISessionSummary):
     pending_task_lane: dict[str, Any] | None = None
     is_branching: bool = False
     active_branch_id: str | None = None
+    # Triage / cockpit header fields
+    client_name: str | None = None
+    asset_name: str | None = None
+    issue_category: str | None = None
+    triage_hypothesis: str | None = None
+    evidence_items: list[dict[str, Any]] | None = None
 
     model_config = {"from_attributes": True}
 
@@ -276,6 +290,16 @@ class QuestionItem(BaseModel):
     """A question the AI needs answered by the engineer."""
     text: str
     context: str = ""
+    options: list[str] | None = None  # quick-reply button labels; null = free-text input
+
+
+class TriageUpdate(BaseModel):
+    """AI-inferred triage metadata returned with chat responses."""
+    client_name: str | None = None
+    asset_name: str | None = None
+    issue_category: str | None = None
+    triage_hypothesis: str | None = None
+    evidence_items: list[dict[str, Any]] | None = None  # appends to existing list
 
 
 class ChatMessageResponse(BaseModel):
@@ -285,6 +309,7 @@ class ChatMessageResponse(BaseModel):
     fork: ForkMetadata | None = None
     actions: list[ActionItem] | None = None
     questions: list[QuestionItem] | None = None
+    triage_update: TriageUpdate | None = None
 
 
 class SaveTaskLaneRequest(BaseModel):
@@ -307,3 +332,24 @@ class AISessionSearchResult(BaseModel):
     created_at: datetime
 
     model_config = {"from_attributes": True}
+
+
+# ── Triage / Cockpit ──
+
+class TriagePatchRequest(BaseModel):
+    """Update triage metadata on a session (incident header fields)."""
+    client_name: str | None = None
+    asset_name: str | None = None
+    issue_category: str | None = None
+    triage_hypothesis: str | None = None
+    evidence_items: list[dict[str, Any]] | None = None
+
+
+class TriagePatchResponse(BaseModel):
+    """Updated triage metadata after a PATCH."""
+    id: UUID
+    client_name: str | None = None
+    asset_name: str | None = None
+    issue_category: str | None = None
+    triage_hypothesis: str | None = None
+    evidence_items: list[dict[str, Any]] | None = None