fix(cron): preserve the invoking computer by default

[just-cameron]

Overlord (agent-c2adbf5c-8419-4211-8cd8-3740db164974)

Summary

• Restore invocation locality for durable Cloud schedules by resolving and verifying the computer that runs letta cron add, then persisting it as target_device_id.
• Keep the explicit precedence: --runner local, --computer <id>, --runner cloud, then the verified active computer.
• Fail without creating a schedule when the active computer or durable Cloud schedule route cannot be verified. The error names the three explicit choices.
• Update bundled scheduling guidance to match the runtime behavior.

Behavior

Before, an untargeted Cloud-agent schedule ran in the managed Cloud sandbox even when it was created from a connected computer. After this change, the default durable schedule targets the verified invoking computer. Explicit --runner cloud remains the opt-in for an untargeted managed-sandbox schedule.

Existing targetless schedules keep their current behavior. This PR does not change server dispatch, offline fallback, or listener acknowledgement semantics.

Test plan

• runCronSubcommand() tests through a real local HTTP boundary for registered default, runtime override, explicit Cloud, explicit computer, local runner, missing schedule routes, unregistered computer, verification failure, and Desktop-local-only identity
• runner and payload unit tests
• prompt and bundled-skill contract tests
• bun run check
• independent adversarial review of the committed diff

👾 Generated with Letta Code

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[just-cameron]

Overlord (agent-c2adbf5c-8419-4211-8cd8-3740db164974)

Superseded by #3665, which is the canonical implementation PR.