feat(setup): first-run site join via CFG_MASTER_DIRECTORY_URL / CFG_MASTER_DIRECTORY_JOIN_KEY

Multi-site join wiring (server + UI landed in theta-directory v2.3.0):

- bootstrap/site-join.js: runs inside the sso-manager container (same
  self-contained rule as bootstrap.js); logs in as the bootstrap admin and calls
  /api/site/join. Idempotent: an already-joined node reports 'already a spoke'.
- setup.sh step 5b: if setup.env sets CFG_MASTER_DIRECTORY_URL +
  CFG_MASTER_DIRECTORY_JOIN_KEY, run the join after the bootstrap. Only honored
  on first run (ensure_config reads setup.env once and ignores it once
  ./config/ exists), so an already-populated directory can never be merged.
- setup.env.example documents both vars.
- lint.yml also node --check's site-join.js.

.github/workflows/lint.yml

@@ -28,7 +28,7 @@ jobs:
         run: shellcheck -S warning setup.sh
 
   bootstrap-syntax:
-    name: Syntax check bootstrap.js
+    name: Syntax check bootstrap scripts
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
@@ -40,7 +40,9 @@ jobs:
           node-version: 22.x
 
       - name: Syntax check
-        run: node --check bootstrap/bootstrap.js
+        run: |
+          node --check bootstrap/bootstrap.js
+          node --check bootstrap/site-join.js
 
       - name: Jump-host LDAP config consistency
         run: node test/check_jump_ldap_tls.js