fix(mcpb): address review feedback on manifest and run.sh

- Use SPDX identifier AGPL-3.0-only (consistent with pyproject.toml)
- Add compatibility.platforms: [darwin, linux] — run.sh uses /bin/sh
  and is not compatible with Windows; this surfaces that at install time
  rather than silently failing to launch
- Improve run.sh fallback: use 'command -v uvx' before exec so a missing
  uvx prints a clear error with install instructions instead of a generic
  shell 'not found' message

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

mcpb/manifest.json

@@ -13,7 +13,7 @@
     "url": "https://github.com/cbcoutinho/nextcloud-mcp-server"
   },
   "homepage": "https://github.com/cbcoutinho/nextcloud-mcp-server",
-  "license": "AGPL-3.0",
+  "license": "AGPL-3.0-only",
   "keywords": ["nextcloud", "notes", "calendar", "contacts", "files", "deck"],
   "server": {
     "type": "python",
@@ -28,6 +28,9 @@
       }
     }
   },
+  "compatibility": {
+    "platforms": ["darwin", "linux"]
+  },
   "tools_generated": true,
   "user_config": {
     "nextcloud_host": {