fix(workspaces): await getAgentByName so deletion actually purges storage

[posthog]

Summary

The agents 0.18.0 upgrade (commit 7e579707) made getAgentByName async — it now returns a Promise. The workspace/account deletion lifecycle helpers called it without await and papered over the type with as unknown as, so callers ended up invoking .purgeForDeletion() / .mergeLinkedAnonymousUser() on a Promise. That threw on every invocation of the resource-purge path. The error was caught and recorded as an operational failure rather than crashing the request, so deletions appeared to succeed while the backing Durable Object storage and R2 files were never purged — leaving orphaned data, storage cost, and a data-retention problem.

This restores the awaited usage that ai/auth.ts and kernel/workspace-kernel-auth.ts already use correctly.

Changes

• Make getUserAIStoreLifecycleAgent / getWorkspaceKernelLifecycleAgent async and await getAgentByName(...), then await them at their call sites.
• Fix the same missing await in ai/user-ai-agents.ts (mergeLinkedAnonymousUser).
• Drop the as unknown as casts that hid the Promise type. The DO namespaces are typed as DurableObjectNamespace<Agent>, so the awaited stub is directly assignable to the narrow lifecycle interfaces — TypeScript now catches this class of missing-await bug.

Why

Every workspace and account deletion silently failed to purge its backing storage after the 0.18.0 upgrade, and the failure was swallowed as a logged operational event — making it a quiet data-retention / orphaned-storage regression.

Test plan

• tsc --noEmit passes with no casts (confirms the awaited stub is assignable to the interfaces, so a future missing await would fail to compile).
• vp lint clean on the changed files.

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[github-actions]

React Doctor found no new issues. 🎉

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