The forwarded host env vars (HIVE_FORGE_URL, HIVE_FORGE_PUBLIC_URL,
HYPERHIVE_HIVE_DOMAIN, ...) were emitted only into the harness
service's own unit environment. Under network isolation that left the
bash-task runner, the matrix daemon, tea-login and interactive shells
without HIVE_FORGE_URL, so `hive-forge` and friends fell back to the
loopback default (localhost:3000) — unreachable from a private netns,
where the in-cluster gateway URL (forge.<domain>) is the only path.
Emit the forwarded set into systemd.globalEnvironment too, which every
unit + shell in the container inherits, so all in-container surfaces
see the same forge/matrix endpoints. The existing harness-service
forward is kept (harmless redundancy). Shared netns is unaffected (the
localhost default still works there).
Adds a regression test asserting the forwarded vars land inside the
globalEnvironment block, and the generated flake parses.
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