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>
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