docs(sync-host-keys): fix stale host-keys/ references in comments and usage

After the clan vars migration all keys are in vars/per-machine/, not
host-keys/. Update:
- File header: "existing clan var is never overwritten" (not host-keys/ file)
- Header --remove/--regenerate description: mention clan vars as primary
- usage() --remove, --regenerate-all-keys, --dry-run text
- cmd_remove/cmd_regenerate_all empty-guard messages
- README.md vars/per-machine/ row: "all deployed hosts" (not "LXC hosts")

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B2EJ4qTsM5KUqhS5c3GAwx

README.md

@@ -73,7 +73,7 @@ nix eval --json .#nixosConfigurations --apply builtins.attrNames | jq -r '.[]'
 | `modules/nix-cache/` | Binary cache and remote builder client/server modules |
 | `modules/installer/` | Auto-installer environment (ISO, also served as PXE netboot) — see `docs/auto-installer.md` |
 | `host-keys/` | Gitignored; only used by the auto-installer environment for pre-seeding SSH host keys before first boot — see `docs/auto-installer.md`. All deployed hosts use clan vars (`vars/per-machine/<target>/openssh/`) instead |
-| `vars/per-machine/` | Clan vars: committed, sops-encrypted SSH host keys for deployed LXC hosts; read by `create-proxmox-resource.sh` at deploy time |
+| `vars/per-machine/` | Clan vars: committed, sops-encrypted SSH host keys for all deployed hosts; read by `create-proxmox-resource.sh` at deploy time |
 | `docs/` | Operational notes for cache, builders, lock updates, boot services, the auto-installer, and Proxmox image builds |
 | `scripts/` | Codex setup, validation, host-key, release-bump, and Proxmox resource helpers |