fix: replace Type.Record with Type.Object for metadata_filters schema

Type.Record(Type.String(), Type.Unknown()) emits patternProperties,
which several provider compat paths strip or reject (Gemini drops
patternProperties; OpenAI strict-tools rejects it). The result was
that metadata_filters disappeared from the schema presented to some
models (e.g. github-copilot/gpt-5.4).

Switch to Type.Object({}, { additionalProperties: true }) so the
parameter renders as a plain open-ended object that survives every
provider's schema sanitizer. Runtime/types unchanged — the handler
still casts to Record<string, unknown>.

Fixes #38

Author: groksrc

tools/search-notes.ts

@@ -26,10 +26,14 @@ export function registerSearchTool(
           }),
         ),
         metadata_filters: Type.Optional(
-          Type.Record(Type.String(), Type.Unknown(), {
-            description:
-              "Filter by frontmatter fields. Supports equality, $in, $gt/$gte/$lt/$lte, $between, and array-contains operators.",
-          }),
+          Type.Object(
+            {},
+            {
+              additionalProperties: true,
+              description:
+                "Filter by frontmatter fields. Supports equality, $in, $gt/$gte/$lt/$lte, $between, and array-contains operators.",
+            },
+          ),
         ),
         tags: Type.Optional(
           Type.Array(Type.String(), {