Add checkpoint types and stop signal for agent suspension

Introduce Checkpoint, CheckpointStore, SuspendedError, AgentRegistry,
and CompletedCall types. Add cooperative stop signal via context.
Export CompletedCall (was unexported completedCall) so checkpoints
can reference completed tool results. Add JSON tags to ToolResult
and ApprovalResult for checkpoint serialization.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/agent/errors.go

@@ -59,7 +59,7 @@ type (
 		inner          *InterruptedError
 		toolCallID     string
 		allToolCalls   []llm.ToolCall
-		completedCalls []completedCall
+		completedCalls []CompletedCall
 	}
 
 	outerLoopState struct {
@@ -69,14 +69,9 @@ type (
 		turns          int
 		allToolCalls   []llm.ToolCall
 		toolCallID     string
-		completedCalls []completedCall
+		completedCalls []CompletedCall
 		innerInterrupt *InterruptedError
 	}
-
-	completedCall struct {
-		toolCallID string
-		result     ToolResult
-	}
 )
 
 func (e *MaxTurnsExceededError) Error() string {