Tighten ctx-suspend plumbing and trim docs

Address review feedback:

- Move ErrSuspendForCheckpoint from checkpoint.go to errors.go
  next to the rest of the agent error declarations; drop the
  colon in the error string so it matches the existing
  `agent run <event>` style used by the supervisor sentinels.
- Replace the inline `outerCtx := ctx; ctx = context.WithoutCancel(ctx)`
  pattern with a small `suspendShield` helper in context.go used
  by coreLoop, resumeWithOpts, and resumeNested. Reads more
  cleanly and stops surfacing the WithoutCancel mechanism at
  every call site.
- Trim the doc comments on Run, RunStreamed, Resume, Restore, the
  ErrSuspendForCheckpoint declaration, and the saveCtx comment in
  restoreNestedSuspended down to the contract bullet.

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

pkg/agent/errors.go

@@ -15,11 +15,16 @@
 package agent
 
 import (
+	"errors"
 	"fmt"
 
 	"go.probo.inc/probo/pkg/llm"
 )
 
+// ErrSuspendForCheckpoint is the cancel cause to use when the caller
+// wants the agent loop to gracefully suspend.
+var ErrSuspendForCheckpoint = errors.New("agent run graceful suspend requested")
+
 type (
 	MaxTurnsExceededError struct {
 		MaxTurns int