refactor(config,cli,server): rename the server: config block to serve:

Introduces the `serve:` host-role block in its RFC-002 shape — `graphs:` is a
served-set list, replacing the single `server.graph` scalar. The legacy
`server:` block is folded into `serve:` under the legacy schema and rejected
under `version: 1` (pointing at the new spelling, noting the scalar->list
change). Serving a true subset (`serve.graphs` with more than one entry) is
rejected with a route-unification hint; one entry (single-graph) and none
(serve all) preserve today's behavior.

Renames the accessors (server_* -> serve_*) and repoints the server boot call
sites; migrates the init scaffold to `serve:`. `servers:` (the remote endpoint
map) is deliberately unaffected — the key scan is exact-match.

crates/omnigraph-cli/src/main.rs

@@ -1579,8 +1579,9 @@ graphs:
     storage: {}
     # bearer_token_env: OMNIGRAPH_BEARER_TOKEN
 
-server:
-  graph: local
+serve:
+  graphs:
+    - local
   bind: 127.0.0.1:8080
 
 defaults: