A briefing page per target site

Signals, loose ends and relayed messages that concern a site, on one page that site's agent can be pointed at (briefings/<site>.html), with the JSON and markdown beside it. Some briefings address several sites at once (the Sovereign AI procurement signal): they appear on each.

Done when: the build makes a page per to_site / waiting_on; briefings/ holds explicit briefs; the newsroom page links them.