marketing: add the scaffolding and positioning source of truth

One voice, one place, so agent-drafted assets stop inventing framing
per artifact. positioning.md carries the wedge, the category-noun rule
(freno-dev/AGENTS.md competitor ban), claims each backed by code, the
honest privacy scoping and the voice/banned-phrasings list. The tree
adds calendar.json, per-channel briefs, drafts → pending → approved →
posted, metrics and listening — with the hard rule up front: approved/
is the only thing a human ever publishes from, nothing posts itself.

Issue: FRE-17

marketing/calendar.json

@@ -0,0 +1,4 @@
+{
+  "$comment": "Scheduled pushes: date, channel, status. Statuses: draft | pending | approved | posted. Move nothing to posted without the artifact living in approved/ first — see positioning.md rule 5.",
+  "pushes": []
+}