feat: support multiple Proxmox instances in single external config

[crack-kitty]

Use Traefik Go template conditionals to support up to 10 Proxmox servers from a single proxmox.yml. Additional instances activate automatically when their env vars are set in .env.external — no repo changes needed to add new servers.

Also silences noisy "Skipped" warnings in traefik-sync-dns for unconfigured conditional instances. Updates pre-commit hook to respect .yamlignore for files with Go template syntax.

more details:

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Summary

• Adds support for up to 10 Proxmox VE instances in a single external-available/proxmox.yml using Traefik Go template conditionals ({{if env "PROXMOXN_ADDRESS"}})
• Additional instances activate automatically when their _HOST_NAME and _ADDRESS env vars are set in .env.external — no repo changes or new YAML files needed
• Silences noisy "Skipped" warnings from make traefik-sync-dns for unconfigured conditional instances
• Updates scaffold template with commented examples for proxmox through proxmox4
• Updates pre-commit hook to respect .yamlignore (needed since Go template conditionals aren't valid YAML)
• Adds "Multi-Instance Services" section to external services documentation

▎ Note: This is a much improved approach over PR #70 which was closed due to potential breaking changes. This PR is fully backwards compatible — the existing single-instance Proxmox
config is unchanged, and the conditional blocks only activate when env vars are explicitly set.

How to use

Add to .env.external:

PROXMOX2_HOST_NAME=proxmox2
PROXMOX2_ADDRESS=192.168.1.51

Then:

make disable-external proxmox && make enable-external proxmox
make restart-service traefik
make traefik-sync-dns

Test plan

• All 325 unit tests pass
• Verify single Proxmox instance still routes correctly (backwards compat)
• Verify additional instances activate when env vars are set
• Verify make traefik-sync-dns picks up new instances without warnings
• Verify unconfigured instances (proxmox4-10) produce no errors in Traefik logs