Continues #718 docs-extraction. Three more blocks moved to
`docs/gateway.md` (which already houses the gateway architecture
story from #775):
1. **Firewall posture (gateway on vs off)** — was a 22-line block
above `networking.firewall = lib.mkIf ...` in hive-c0re.nix.
Trimmed to 3-line ref. New `docs/gateway.md::Firewall posture
(host-level)` section covers the gateway-on / gateway-off
trade-off + why dashboard port stays loopback-only.
2. **`HIVE_FORGE_URL` loopback rationale** — was a 14-line block
above the env-var assignment. Trimmed to 5-line ref. New
`docs/gateway.md::HIVE_FORGE_URL: loopback for in-cluster,
sub-domain for the operator` section covers the in-cluster vs
external split + why agent containers can't use the sub-domain.
3. **hive-forge container shape** — was a 15-line top-of-`config`
block in hive-forge.nix explaining the nixos-container + host
netns choices. Trimmed to 4-line ref. New
`docs/gateway.md::hive-forge container shape` section captures
the same content with state-dir + wipe-via-destroy notes.
Net: hive-c0re.nix -29 lines, hive-forge.nix -11 lines, gateway.md
+44 lines. Same pattern as #782 (first pass) per iris's #10114
guidance — substantive WHY moves to docs as named sub-paragraphs,
in-code shrinks to `// see docs/<file>::<section>` refs.
Verified: `nix eval` on agent-base toplevel still resolves
cleanly; firewall posture unchanged (still 0 ports opened in the
gateway-on case + the same 8100..8999 range in the gateway-off
case).
Continues #718. Follow-up batches: remaining harness-base.nix
blocks, nix/docs/default.nix, nix/assets.nix, nix/templates/weston-vnc.nix.
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