fix(proxmox): observe live VM config without false drift

[astrojerms]

Changes

• Accept numeric JSON strings in Proxmox VM configuration and propagate configuration-read failures.
• Observe native VM settings instead of partial runtime inventory.
• Apply provider-owned drift comparison consistently across Get, List, Watch, and reconciliation, preserving real drift while excluding creation-only and write-only inputs.
• Add regression coverage for decoding, configuration outages, named device matching, and automatic reconciliation; update documentation.
• Satisfy modernization checks with allocation-free string iteration and standard slice helpers, including one existing graph expression.
• Synchronize the existing GitOps periodic pull test cleanup so its temporary repository is not removed during in-flight git commands; no GitOps production API changes.

Verification

• All six GitHub CI jobs passed on the final head: Build, Test (including full root/plugin race suites), Lint, Vet & Staticcheck, Format Check, and Modernize Check.
• CI: https://github.com/astrojerms/openctl/actions/runs/37862783816
• Existing periodic-pull scenario passed 20 race-enabled local runs after cleanup synchronization.
• Local repository/plugin tests, focused race tests, and full build passed; Proxmox, operation graph, and controller server regressions passed after modernization changes.
• Read-only Get/List against the homelab dev-system VM: zero drift; correct 8 vCPUs, 10 GiB RAM, and 300 GiB disk.
• No infrastructure configuration was changed during verification; temporary controller stopped afterward.