Adds set_boot_warning() to stats/warnings.rs: a one-shot variant of
set_warning() for steps that run once at hive-c0re boot (no periodic
retry to observe a later success and clear the banner via WarningGuard
drop). It forgets the guard, leaking it for the life of the process —
the banner clears on the next hive-c0re restart, which is also when a
config/environment fix would take effect.
Wires every remaining tracing::warn!()-and-forget site in
forge::ensure_all() to also raise a boot warning: core user/token
provisioning, per-org seeding, the operators-team grant per org, the
core/meta repo, shared docs repo, knowledge repo (+ its local clone),
both avatar uploads, and the final nixos-container list failure.
Split the core_token.is_some() body of ensure_all() into a new
ensure_all_orgs_and_repos() helper to stay under clippy's
function-length limit.
Deliberately out of scope: sync_agent() (shared with rebuild_agent(),
different blast radius) and ci_runner::ensure_ci_runner_registered()
(its own retry logic) — matrix::ensure_all() got the same slice-by-
slice treatment.
