#38 Hide empty sections from header nav

Context: Owner QA — nav shows tabs for empty sections (e.g. tasks/releases with nothing in them). Dead tabs are noise. Tasks:

1. At build: a section tab renders in the header ONLY if it has content. Empty = issues none AND pulls none? NO — issues/pulls/new always stay (they carry the + submit path). Hide when empty: tasks (0 tasks), releases (0 releases), wiki (0 pages), ci? (ci always has env/steps context — keep ci always). commits/code/feed/api/agents always stay.
2. Direct URLs to hidden sections still build (deep links keep working); only the NAV hides the tab.
3. Empty states on index pages unchanged ("No X yet" + submit links where relevant).
4. Tests: empty-forge fixture asserts tasks/releases/wiki tabs absent from header but their pages still exist; full forge asserts all tabs present. Success rubric:

• Demo (empty tasks? check) header shows only populated tabs; hidden pages still 200. Swarm: Owl, small. Branch m6/hide-empty.