Move ErrSuspendForCheckpoint to pkg/agent

The sentinel is part of the agent cancellation contract — the only
caller that needs it (the supervisor) imports pkg/agent already, so
keeping it next to SuspendedError prevents the upward dependency
that would arise if any future agent.Run caller wanted to trigger
graceful suspend. Update pkg/probo/agent_run_handler.go to
reference agent.ErrSuspendForCheckpoint.

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

pkg/agent/checkpoint.go

@@ -16,10 +16,21 @@ package agent
 
 import (
 	"context"
+	"errors"
 
 	"go.probo.inc/probo/pkg/llm"
 )
 
+// ErrSuspendForCheckpoint is the recommended cancel cause for callers
+// who want the agent loop to gracefully suspend (build a checkpoint
+// and return *SuspendedError) rather than treat the cancellation as a
+// silent close. The agent loop only inspects ctx.Err(); any cancel
+// cause produces a graceful suspend, but using this sentinel makes
+// the intent explicit and lets supervisors distinguish a user-driven
+// cancel from infrastructure-level causes (lease loss, heartbeat
+// failure) when they inspect context.Cause(ctx).
+var ErrSuspendForCheckpoint = errors.New("agent run: graceful suspend requested")
+
 type (
 	AgentStatus string