Switch per-agent gateway upstreams from TCP loopback to unix-domain
socket when the agent has opted in via `hyperhive.web.useUnixSocket`
(#822). Coexists with the TCP path during rollout.
Changes:
- New `agentSocketsFile` option (default
`/var/lib/hyperhive/agent-sockets.json`) — c0re writes the map
there via `hive_c0re::agent_sockets::write` (#809).
- `agentSocketsTable = lib.importJSON ...` (graceful empty when
file missing).
- `agentUpstreamFor name port` picks `http://unix:<path>:/` when the
socket has a JSON entry AND the file exists at eval time; else
`http://127.0.0.1:<port>/`. Path-exists gate guards against
c0re's blanket-emit shape during the canary window (agents in
`agent-sockets.json` who haven't actually flipped have no
bound socket on disk → fall back to TCP). Damocles will ship a
`.bound` marker filter on the c0re side (#784 step 2d
follow-up); once that's in, the path-exists check is redundant
but harmless. Step 4 drops it entirely along with the TCP
fallback.
- `containers.hive-gateway.bindMounts."/run/hive-agent"` —
read-only, unconditional. Inert when no agents have opted in.
Required so nginx inside the gateway container can `connect(2)`
to the per-agent sockets damocles's #813 bind-mounts into agent
containers at the same paths.
Docs:
- `docs/gateway.md::Per-agent UDS upstream (#784)` — full rollout
flow, subdir-bind rationale (damocles #813), eval-time gate
explainer, step 4 drop plan.
`nix flake check` clean; `nix fmt` clean.
Canary plan: once #822 (`useUnixSocket` option) lands + this PR
merges, manager flips atlas's agent.nix to `useUnixSocket = true`
via the config-update flow. End-to-end validation against atlas
before broader rollout.
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