P2: Reject empty run payloads

[vamgan]

Problem

agentdispatch run can dispatch without either --instruction or --command.

Relevant code:

• src/index.ts:85-86
• src/index.ts:196-227

In that case createDispatchRequest() builds an agent.run request with no meaningful payload.

Impact

Adapters/workers receive empty tasks and fail later with less actionable errors.

Suggested fix

Validate before dispatch:

• require --instruction for agent.run
• require --command for command.run
• define or reject behavior when both are provided

Acceptance criteria

• Missing payload fails fast with a helpful CLI error.
• Tests cover missing payload and both-payload cases.