Remove requirement of repository.id from server.json (#285)

Make repository.id optional by:
- Removing 'id' from required fields in JSON schema
- Adding omitempty tags to Go model structs 
- Updating examples to show simpler format without id

This fixes publishing failures where the publisher tool doesn't 
generate repository.id but the schema requires it.

Fixes #179

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Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: adam jones <domdomegg@users.noreply.github.com>

docs/server-json/examples.md

@@ -9,8 +9,7 @@
   "status": "active",
   "repository": {
     "url": "https://github.com/modelcontextprotocol/servers",
-    "source": "github",
-    "id": "abc123de-f456-7890-ghij-klmnopqrstuv"
+    "source": "github"
   },
   "version_detail": {
     "version": "1.0.2"

docs/server-json/schema.json

@@ -8,8 +8,7 @@
       "type": "object",
       "required": [
         "url",
-        "source",
-        "id"
+        "source"
       ],
       "properties": {
         "url": {