proxmox: deployment files, sample, user guide and doc rows

The optional proxmox-token Secret (token, ca.pem) mounted at
/etc/proxy-operator/proxmox, make proxmox-token-secret with content
checks that never print the token, a commented proxmox-home entry in
both providers configs, and config/samples/proxy_proxmox.yaml.
TestLoadConfig_proxmoxSampleEntry loads the commented entry from both
files, and a second test ties the sample Proxy's placement to it.

docs/proxmox-provider.md is the user guide: prerequisites and privileges,
cluster setup, config reference, placement, what lands on Proxmox,
logging, tracing, timeouts, health check, error classes, known gaps and
testing. providers-existing.md gains the proxmox column and section;
architecture.md, the README, providers.md and CLAUDE.md gain their rows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -226,3 +226,6 @@ A lightweight `check` job (`go vet` / `go build` / `go test -short`) gates the b
 - `+kubebuilder:rbac` markers are package-level. Keep them in their own comment block,
   never inside a type's or function's doc comment: controller-gen silently ignores
   them there. After `make manifests`, check `git diff config/rbac/role.yaml`.
+- `make test-integration-proxmox` runs the proxmox provider's integration tests against
+  a real Proxmox VE host. They skip unless `PROXMOX_TEST_URL`, `_TOKEN_FILE`, `_NODE`,
+  `_TEMPLATE_VMID` and `_ISO_STORAGE` are set; see docs/testing.md for the full env block.