Replace third-party Slurm exporter with a DeepOps-owned implementation

[dholt]

Summary

• Replace the third-party Slurm exporter (unmaintained since 2024; required a build-time parser patch as of fix: build the Slurm exporter image locally with a current base #1365) with a small DeepOps-owned exporter maintained in this repository: standard-library-only Go, no external modules, built by the role from in-repo source — the external clone is gone entirely.
• Emits exactly the metric names the shipped Grafana dashboard consumes (slurm_nodes_*, slurm_queue_*, slurm_scheduler_*) plus the aggregate slurm_cpus_* gauges, with all series always present (idle clusters omit backfill statistics from sdiag; the gauges still emit as 0).
• Locale- and truncation-safe Slurm client invocations by construction; collectors degrade gracefully (slurm_exporter_collector_errors) instead of crash-looping; 8 unit tests including fixtures for Slurm 26.05 output shapes.
• Intentionally dropped relative to the previous exporter, as nothing in this repository consumes them: per-node, per-user, and fair-share series (the sshare bind mount is removed accordingly). Sites needing them can set slurm_exporter_build_image: false and supply their own image.

Validation

• go vet, gofmt, and the unit suite pass; on a Slurm-less host the exporter serves HTTP 200 with collector_errors 4 rather than failing.
• Full role lint (0 failures) and playbook syntax check.
• Live single-GPU deployment (Ubuntu 22.04, Slurm 26.05.1 via a fresh slurm-cluster.yml run): image 2.0.0 built from in-repo source by the role, service active with NRestarts=0, /metrics HTTP 200 serving 31 slurm_* series covering the dashboard contract completely, collector_errors 0, and automatic recovery after systemctl restart docker.

Notes

• Follows the maintainer decision to prefer small owned components over stale third-party dependencies. The upstream parser fix from fix: build the Slurm exporter image locally with a current base #1365 remains submitted upstream for other users of that exporter.