Slurm adapter: real HPC cluster integration

## Description

`adapters/slurm/src/main.rs` (135 lines) has a CLI scaffold and config struct but no actual Slurm API integration. The `status` command returns "not yet implemented".

## Requirements

- Connect to Slurm head node via slurmrestd REST API or SSH+sbatch fallback
- Advertise aggregate cluster capacity to World Compute broker
- Dispatch incoming tasks as Slurm batch jobs (sbatch)
- Monitor job status via sacct/squeue
- Report results back to World Compute data plane
- Handle Slurm-specific errors (PENDING, TIMEOUT, NODE_FAIL)
- Implement install/configure/status CLI commands

## Success Criteria

- [ ] Adapter connects to real Slurm cluster
- [ ] Jobs dispatched via sbatch and results collected
- [ ] Cluster capacity advertised accurately
- [ ] Error handling for common Slurm failure modes
- [ ] Integration test on real 2+ node Slurm testbed
- [ ] Adapter appears as aggregate node in `worldcompute cluster peers`

## Testing (Principle V)

- Deploy on real Slurm cluster (even 2-node test setup)
- Submit SHA-256 test job → verify correct result
- Simulate node failure → verify job rescheduled
- Verify resource reporting matches `sinfo` output