devices: log Proxmox pool listing when a declared pool can't be validated

Summary

• Diagnoses the "pool klab parsed correctly but Rax says it doesn't exist" report: GET /pools on Proxmox silently filters out any pool the API token lacks Pool.Audit on — returns 200 with an incomplete list, not an error — so a token that can manage VMs but wasn't granted pool-audit rights makes Rax wrongly block on "pool does not exist".
• Logs Proxmox's actual pool listing whenever a device's declared pool isn't found in it, so this is diagnosable from Rax's own logs instead of guessing.
• README now calls out the Pool.Audit requirement explicitly under the Proxmox-token prerequisite section.

Test plan

• go build ./... && go vet ./... && go test ./... all green in the standard Docker toolchain
• Operator: check Rax's logs next time this blocker fires — if the logged pool list is missing klab, grant the Rax Proxmox token Pool.Audit and re-check

🤖 Generated with Claude Code

https://claude.ai/code/session_01U9buZPRQy7YXLodkSPFFik