feat: provider manager, cancellation, session summaries (store+agent)

internal/llm/llm.go

@@ -24,6 +24,8 @@ type Message struct {
 	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
 	ToolCallID string     `json:"tool_call_id,omitempty"`
 	Name       string     `json:"name,omitempty"`
+	// Cancelled marks a partial assistant answer that was stopped by the user.
+	Cancelled bool `json:"cancelled,omitempty"`
 }
 
 // ToolSpec describes a callable tool using JSON Schema parameters.