feat(MR-656): inline query strings in CLI and HTTP server

CLI:
- Add -e / --query-string <STRING> to omnigraph read and omnigraph change
- Exactly one of --query, --query-string, --alias is required (3-way XOR)
- Empty --query-string is rejected with a clear error

HTTP:
- New POST /query (read-only, clean field names: query/name/params/branch/snapshot)
- Mutations on /query are rejected with 400 -- use POST /change instead
- ChangeRequest fields polished: query (alias query_source), name (alias query_name)
- POST /read and POST /change remain byte-compatible for existing clients

Tests:
- cli.rs: -e happy-path on read/change, mutex error vs --query, empty -e rejected
- system_local.rs: inline -e read and -e change exercise the local flow
- system_remote.rs: inline -e read/change over HTTP plus direct /query 200/400
- server.rs: /query 200, /query 400 on mutation, /change legacy field alias
- openapi.rs: new /query path, QueryRequest schema, ChangeRequest field-name polish

Docs: cli.md (-e examples), cli-reference.md (read/change rows), server.md (/query)
Co-Authored-By: Ragnor Comerford <ragnor.comerford@gmail.com>

crates/omnigraph-server/tests/openapi.rs

@@ -159,6 +159,7 @@ const EXPECTED_PATHS: &[&str] = &[
     "/healthz",
     "/snapshot",
     "/read",
+    "/query",
     "/export",
     "/change",
     "/schema",
@@ -278,6 +279,7 @@ const EXPECTED_SCHEMAS: &[&str] = &[
     "BranchMergeRequest",
     "ChangeOutput",
     "ChangeRequest",
+    "QueryRequest",
     "CommitListOutput",
     "CommitOutput",
     "ErrorCode",
@@ -368,13 +370,65 @@ fn read_output_schema_has_expected_fields() {
 
 #[test]
 fn change_request_schema_has_expected_fields() {
+    // Canonical field names on the wire are now `query` and `name`. The
+    // schema descriptions document `query_source` and `query_name` as
+    // legacy deserialization aliases for backward compatibility.
     let doc = openapi_json();
     let schema = &doc["components"]["schemas"]["ChangeRequest"];
     let props = schema["properties"].as_object().unwrap();
-    assert!(props.contains_key("query_source"));
-    assert!(props.contains_key("query_name"));
+    assert!(props.contains_key("query"));
+    assert!(props.contains_key("name"));
     assert!(props.contains_key("params"));
     assert!(props.contains_key("branch"));
+    let query_desc = schema["properties"]["query"]["description"]
+        .as_str()
+        .unwrap_or_default();
+    assert!(
+        query_desc.contains("query_source"),
+        "expected `query` description to mention the legacy `query_source` alias, got: {query_desc}"
+    );
+}
+
+#[test]
+fn query_request_schema_has_expected_fields() {
+    let doc = openapi_json();
+    let schema = &doc["components"]["schemas"]["QueryRequest"];
+    let props = schema["properties"].as_object().unwrap();
+    assert!(props.contains_key("query"));
+    assert!(props.contains_key("name"));
+    assert!(props.contains_key("params"));
+    assert!(props.contains_key("branch"));
+    assert!(props.contains_key("snapshot"));
+}
+
+#[test]
+fn query_request_query_is_required() {
+    let doc = openapi_json();
+    let schema = &doc["components"]["schemas"]["QueryRequest"];
+    let required: Vec<&str> = schema["required"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|v| v.as_str().unwrap())
+        .collect();
+    assert!(required.contains(&"query"));
+}
+
+#[test]
+fn openapi_query_is_post() {
+    let doc = openapi_json();
+    assert!(doc["paths"]["/query"]["post"].is_object());
+}
+
+#[test]
+fn query_endpoint_documents_mutation_400() {
+    let doc = openapi_json();
+    let four_hundred = &doc["paths"]["/query"]["post"]["responses"]["400"];
+    let description = four_hundred["description"].as_str().unwrap_or_default();
+    assert!(
+        description.contains("mutations") || description.contains("POST /change"),
+        "expected /query 400 response to mention mutation rejection, got: {description}"
+    );
 }
 
 #[test]