record ADR-0039: site.yaml, and only its site level

The human asked for YAML site config alongside the -base flag, which is the
trigger the parked settings cascade was waiting for — but only for its top level.
So this adopts the site level and leaves section and bundle resolution parked,
with their trigger unchanged: the first section-level override with a reader.
The parked idea now says so, rather than reading as untouched.

Absolute URLs are the forcing function. A canonical link, an hreflang, an
OpenGraph tag and a sitemap entry all need the site's own origin, which the engine
cannot infer from a request it may be serving behind any proxy.

Precedence stated in the ADR: -base beats the file, so a staging host needs no
edit to content.

ideas/deferred-decisions.md

@@ -57,6 +57,10 @@ new field on every type.
 
 ## Settings cascade
 
+Status: partly adopted → the **site** level is built as `site.yaml` (ADR-0039). The section and bundle levels
+stay parked: frontmatter covers a bundle, and the trigger for the rest is unchanged — the first section-level
+override with a reader.
+
 Deferred because frontmatter alone covers the MVP; a cascade earns itself when a section-wide policy is actually wanted.
 
 ## Settings cascade: site → section → bundle