feat(multi-site): spoke.env.example + CFG_PUBLIC_DOMAIN

A dedicated spoke.env for the join-a-cluster vars (CFG_MASTER_DIRECTORY_URL/
_JOIN_KEY, CFG_SPOKE_NO_INBOUND/_PUBLIC_HOST, CFG_PUBLIC_DOMAIN), split out
of setup.env purely for clarity -- setup.env still has every option and
keeps working as a single file if that's preferred. setup.sh reads both
(setup.env first, spoke.env layered on top so its values win), same
first-run-only rule as setup.env already had.

Also adds CFG_PUBLIC_DOMAIN (documented in MULTI_SITE_SPEC.md §4 but never
actually wired into setup.sh): an inbound spoke/standalone site's own public
web domain, independent of CFG_DOMAIN (the shared LDAP identity namespace,
which must stay identical across every site). Unset behaves exactly as
before -- hostnames derive from CFG_DOMAIN like any standalone install.

docs/MULTI_SITE_SPEC.md

@@ -256,7 +256,7 @@ See [`AGENT_LOCAL_DISCOVERY_SPEC.md`](./AGENT_LOCAL_DISCOVERY_SPEC.md) — split
 | Join key issuance + one-time directory adoption | **Shipped** — `/api/site/join-keys`, `/api/site/export`, `/api/site/join`, fresh-install-gated (v2.2.0–v2.3.0) |
 | Spoke read-only enforcement | **Shipped** — directory-write routes 403 toward the master once joined (v2.3.0) |
 | WAN health check | **Shipped** — `/api/site/ping`, live in the Master Site modal (v2.2.0–v2.3.0) |
-| `setup.env` / `setup.sh` join wiring | **Shipped** — `CFG_MASTER_DIRECTORY_URL` / `CFG_MASTER_DIRECTORY_JOIN_KEY`, `bootstrap/site-join.js` (theta-suite v2.2.0) |
+| `setup.env` / `setup.sh` join wiring | **Shipped** — `CFG_MASTER_DIRECTORY_URL` / `CFG_MASTER_DIRECTORY_JOIN_KEY`, `bootstrap/site-join.js` (theta-suite v2.2.0). Also readable from a dedicated `spoke.env` (`spoke.env.example`, layered on top of `setup.env`) for operators who want join-a-cluster config kept separate from the rest of first-run setup. |
 | Continuous/live replication (vs. one-time export-on-join) | **Shipped** (`sso-manager-node`) — a spoke registers its own endpoint at join time (`POST /api/site/spokes`), and every successful master catalog write fires a fire-and-forget push (`utils/site_replicate.js`) at every registered spoke, which re-pulls a fresh export. Verified end-to-end in `docker-compose.multisite-e2e.yml`. |
 | Identical-directory signing key | **Shipped** — `POST /api/site/export` includes the master's agent-signing key; a spoke adopts it via `agent_keys.adopt()` on join and every resync. OpenBao secret replication *beyond* this one key is still not built. |
 | Coordinated master promotion (demote the old master as one action) | **Shipped** — `POST /api/site/demote` + `site-promote`'s handoff logic. Fixed two real pre-existing bugs while wiring this in: `site-promote`'s god_admin check read a `req.user.groups` field nothing ever populated (permanently 403'd for everyone), and the read-only write-gate 403'd `site-promote` itself before the handler could run. |

docs/sso/multi-site.md

@@ -38,8 +38,13 @@ between the two.
    admin, no enrolled agents), either:
    - Paste the master's URL and the join key into the Master Site modal's
      **Join an Existing Site** form, or
-   - Set `CFG_MASTER_DIRECTORY_URL` / `CFG_MASTER_DIRECTORY_JOIN_KEY` in
-     `setup.env` before the first `./setup.sh` run.
+   - Set `CFG_MASTER_DIRECTORY_URL` / `CFG_MASTER_DIRECTORY_JOIN_KEY` before
+     the first `./setup.sh` run -- either in `setup.env` (which has every
+     option), or in a dedicated `spoke.env` (`cp spoke.env.example spoke.env`)
+     if you'd rather keep join-a-cluster config separate from the rest of the
+     stack's setup. Both are read; `spoke.env`'s values win on a conflict.
+     No public IP on this site at all? `spoke.env.example` also covers the
+     no-inbound relay vars (`CFG_SPOKE_NO_INBOUND`/`CFG_SPOKE_PUBLIC_HOST`).
 3. The spoke pulls the master's full export (LDAP tree, resource catalog,
    agent-signing key) and adopts it, then registers its own reachable URL
    with the master so it can receive live updates going forward.