fix: commit uncommitted native JSON type changes on master

These changes (HashMap<String, String> → HashMap<String, Value> for front matter,
json_value_to_yaml_scalar, and oneOf schema for front_matter) were left uncommitted
on master after a previous merge, blocking the cherry-pick step of story 509's merge.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/http/mcp/mod.rs

@@ -664,9 +664,15 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                             },
                             "front_matter": {
                                 "type": "object",
-                                "description": "Arbitrary YAML front matter key-value pairs to set or update",
+                                "description": "Arbitrary YAML front matter key-value pairs to set or update. Values may be strings, booleans, integers, numbers, or arrays (e.g. [490, 491]).",
                                 "additionalProperties": {
-                                    "type": "string"
+                                    "oneOf": [
+                                        {"type": "string"},
+                                        {"type": "boolean"},
+                                        {"type": "integer"},
+                                        {"type": "number"},
+                                        {"type": "array"}
+                                    ]
                                 }
                             }
                         },