fix(sites): remove stale domain links on sync

[rissrice2105-agent]

Summary

• treat the domain database as authoritative during startup sync
• remove entries from the domain symlink farm when they are absent from AllDomains()
• rebuild every current mapping after stale entries are removed
• add a regression for a stale domain-directory entry

Closes #109

Testing

• go test ./internal/sites -run TestSyncRemovesStaleDomainEntries -count=1
• go vet ./internal/sites
• Linux internal/sites test binary compilation
• gofmt verification
• git diff --check

The full Windows package run reaches the repository's existing symlink integration test but cannot create symlinks without the Windows privilege; the new regression and Linux compilation both pass.