feat(tmpl): {{domain_name}} placeholder — materialize on diff/apply, parameterize on snapshot

Adds internal/tmpl with Materialize (template placeholder -> zone name) and
Parameterize (zone name -> placeholder, the inverse used by the
template-from-zone snapshot). service.resolve now materializes the template
against DomainRef.ZoneName before diffing, so one template can be reused
across domains. LoadDomainFull (source query + hand-edited sqlc output, since
sqlc is not installed) now also selects zone_name to populate it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/store/queries/domains.sql

@@ -23,7 +23,7 @@ SELECT * FROM domains WHERE project_id = $1 ORDER BY created_at;
 DELETE FROM domains WHERE id = $1 AND project_id = $2;
 
 -- name: LoadDomainFull :one
-SELECT d.zone_id, a.provider, a.secret_enc, t.doc
+SELECT d.zone_id, d.zone_name, a.provider, a.secret_enc, t.doc
 FROM domains d
 JOIN provider_accounts a ON a.id = d.provider_account_id
 LEFT JOIN templates t ON t.id = d.template_id