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>
