feat(ai): session harness contract — incomplete, run_id, confirm bind

Add docs/16 for session state machine, incomplete/resume, queue semantics,
and agent-loop/harness/MCP gap review. Wire RunOutcome on max-rounds,
run_id through audit, TrimMessages/Redact hooks, ConfirmBindHash on write
confirm path; session file skeleton without HTTP yet. No push.

server/internal/ai/agent_stream.go

@@ -12,7 +12,8 @@ const (
 	AgentEventStep      = "step"
 	AgentEventDelta     = "delta"
 	AgentEventDone      = "done"
-	AgentEventError     = "error"
+	AgentEventError      = "error"
+	AgentEventIncomplete = "incomplete"
 )
 
 // AgentQueryResult 可供「在结果区打开」的行列结果。
@@ -26,15 +27,20 @@ type AgentQueryResult struct {
 
 // AgentEvent 流式推送的一项。
 type AgentEvent struct {
-	Type    string            `json:"type"`
-	Step    *AgentStep        `json:"step,omitempty"`
-	Delta   string            `json:"delta,omitempty"`
-	Content string            `json:"content,omitempty"`
-	SQL     string            `json:"sql,omitempty"`
-	Steps   []AgentStep       `json:"steps,omitempty"`
-	Rounds  int               `json:"rounds,omitempty"`
-	Error   string            `json:"error,omitempty"`
-	Result  *AgentQueryResult `json:"result,omitempty"`
+	Type             string            `json:"type"`
+	Step             *AgentStep        `json:"step,omitempty"`
+	Delta            string            `json:"delta,omitempty"`
+	Content          string            `json:"content,omitempty"`
+	SQL              string            `json:"sql,omitempty"`
+	Steps            []AgentStep       `json:"steps,omitempty"`
+	Rounds           int               `json:"rounds,omitempty"`
+	Error            string            `json:"error,omitempty"`
+	Result           *AgentQueryResult `json:"result,omitempty"`
+	RunID            string            `json:"runId,omitempty"`
+	Outcome          string            `json:"runOutcome,omitempty"`
+	IncompleteReason string            `json:"incompleteReason,omitempty"`
+	Resumable        bool              `json:"resumable,omitempty"`
+	StepsBudget      int               `json:"stepsBudget,omitempty"`
 }
 
 // AgentEmitter 可选；nil 时仅返回最终 AgentRes（非流式 fallback）。