feat(marketing): HoM feature-spotlight X drafts + brand-voice charter (v0.18.0 B)

The Head of Marketing now markets features, not just releases: a default-off
x_feature_spotlight loop periodically spawns the HoM to investigate what shipped
(CHANGELOG, feature flags, docs/map, KB) and draft ONE held marketing post via
propose_feature_spotlight, reviewed in the X post queue.

- New x_feature source (distinct from x_post, fixing panel mislabeling) + a
  panel Feature-spotlight branch.
- brand_voice column on company_goals (migration 061, single head) as the
  CEO-editable voice source, surfaced in Settings and injected into the HoM
  briefing; a VOICE GUIDE baseline in head-marketing.md.
- propose_feature_spotlight verb (HoM-only), mirroring propose_roadmap.

Gated by x_feature_spotlight_enabled (default off; flag-off dormancy proven).
Also fixed two real bugs found mid-build: company_goals API schemas dropped
brand_voice on GET/PUT; the live charter UI is goals-tab.tsx, not the unmounted
company-goals-card.tsx. Full suite green (2935); migration single-head verified.

panel/src/components/settings/feature-flags-card.tsx

@@ -62,6 +62,8 @@ const FLAG_DESCRIPTIONS: Record<string, string> = {
     "Draft release-announcement posts for the X (Twitter) account — every draft is held in a queue for you to edit, approve, or reject; nothing posts automatically, and it stays inert until you set credentials in the X card below.",
   x_replies_enabled:
     "Also poll X mentions and draft replies (still held for your approval — nothing auto-replies). Off by default: reading mentions needs a paid X API tier, so leave this off if you only want release posts.",
+  x_feature_spotlight_enabled:
+    "Periodically spawn the Head of Marketing to investigate RoboCo's shipped features and draft a spotlight post for an under-publicized capability — held in the same X post queue below for your approval. Needs x_engine_enabled and X credentials.",
   roadmap_engine_enabled:
     "Weekly: the Product Owner explores the company's projects and proposes a themed cycle of 3-7 roadmap items — you approve or reject each one individually; approved items land in the backlog and nothing auto-starts.",
   fable_mode_enabled: