Response collection and storage

[jorisvilardell]

Context

Receive execution results from agent and store them in the job.

Acceptance Criteria

• Backend waits for agent response (with timeout)
• Parses { exit_code, stdout, stderr }
• Updates job with results
• Sets job status to Done or Error
• Client can retrieve results via `GET /status/{id}
• Clean up VM after handled

Related Code:**

• src/main.rs - get_status() handler
• src/main.rs - AgentExecuteResponse struct
• src/main.rs - Job TTL cleanup task