Add GET /queries stored-query catalog endpoint

List a graph's mcp.expose stored queries as a typed tool catalog so a client
(the MCP server) can register them as tools without fetching .gq source.
Each entry carries name, MCP tool_name, description/instruction, a
read/mutate flag, and decomposed typed params (kind enum: string|bool|int|
bigint|float|date|datetime|blob|vector|list, plus item_kind for lists and
vector_dim) — so the consumer builds an input schema with a closed match and
never re-parses omnigraph type spelling. I64/U64 are bigint (string on the
wire): a JSON number loses precision past 2^53 and the engine already accepts
decimal strings.

Read-gated (works in default-deny; the catalog is graph-wide, authorized
against main). NOT Cedar-filtered per query yet — a reader can list a query
whose invoke_query they lack (documented gap until per-query authz lands);
invocation stays invoke_query-gated + deny==404.

- api: QueriesCatalogOutput / QueryCatalogEntry / ParamDescriptor / ParamKind
  + query_catalog_entry (reuses PropType::from_param_type_name; scalar_kind is
  exhaustive, so a new ScalarType is a compile error here until catalogued).
- GET /queries route in per_graph_protected (→ /graphs/{id}/queries in multi
  mode); OpenAPI regenerated; path allowlists updated.
- Tests: projection unit (every kind, list, vector, nullable, mutation,
  empty) + handler (exposed-only filter, read-gate probe-oracle, empty
  registry).

crates/omnigraph-server/tests/openapi.rs

@@ -168,6 +168,7 @@ const EXPECTED_PATHS: &[&str] = &[
     "/export",
     "/change",
     "/mutate",
+    "/queries",
     "/queries/{name}",
     "/schema",
     "/schema/apply",
@@ -702,6 +703,7 @@ fn protected_endpoints_reference_bearer_token_security() {
         ("/read", "post"),
         ("/change", "post"),
         ("/schema/apply", "post"),
+        ("/queries", "get"),
         ("/queries/{name}", "post"),
         ("/ingest", "post"),
         ("/export", "post"),