feat(vercel-sandbox,sandbox): introduce keepLast parameters for SDK/CLI (under feature flag)

[marc-vercel]

Introduce keepLast snapshot retention and forceDelete for the SDK and CLI. These changes are gated behind a feature flag until we agree that this is the correct contract.

CLI

sandbox create and the new sandbox config keep-last subcommand configure a retention policy that keeps only the N most recent snapshots. By default, evicted snapshots are deleted immediately; pass --soft-evict to let them fall back to the sandbox's default expiration instead.

> sandbox create --keep-last 3
> sandbox create --keep-last 3 --keep-last-for 7d
> sandbox create --keep-last 5 --keep-last-for 30d --soft-evict

> sandbox config keep-last my-sandbox 3
> sandbox config keep-last my-sandbox 3 --for 7d --soft-evict
> sandbox config keep-last my-sandbox --clear
> sandbox config list my-sandbox     # includes a new "Keep last" row

sandbox snapshots delete supports --force / -f to delete a snapshot that is currently the sandbox's currentSnapshotId (server rejects the delete without it when the flag is on):

> sandbox snapshots delete snap_abc            # fails if the snapshot is in use
> sandbox snapshots delete snap_abc --force    # succeeds

SDK

Sandbox.create and Sandbox.update accept a new optional snapshotKeepLast field.

const sandbox = await Sandbox.create({
  snapshotKeepLast: {
    count: 3,
    expiration: 7 * 24 * 60 * 60 * 1000,   // ms, 0 = never. Falls back to snapshotExpiration when omitted.
    deleteEvicted: true,                    // default true
  },
});

On Sandbox.update, pass null to remove the policy:

await sandbox.update({ snapshotKeepLast: null });

snapshot.delete accepts forceDelete to bypass the server's in-use check where we fail if the latest snapshot is being used by a sandbox (and is the last snapshot):

await snapshot.delete({ forceDelete: true });

[vercel]

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Project	Deployment	Actions	Updated (UTC)
sandbox	Ready	Preview, Comment, Open in v0	Apr 27, 2026 3:18pm
sandbox-cli	Ready	Preview, Comment, Open in v0	Apr 27, 2026 3:18pm
sandbox-sdk	Ready	Preview, Comment, Open in v0	Apr 27, 2026 3:18pm
sandbox-sdk-ai-example	Ready	Preview, Comment, Open in v0	Apr 27, 2026 3:18pm
workflow-code-runner	Ready	Preview, Comment, Open in v0	Apr 27, 2026 3:18pm

[QuiiBz]

We could use SandboxMetaData['keepLastSnapshots'] here and everywhere else where we duplicate the types definition, but I think we should instead extract into its own type (basically everything line 119-134) so we keep the TSDoc everywhere