Remove package_canonical (#59)

I originally introduced `package_canonical` into the API schema because,
at the time, we were thinking to "pin" the version of the MCP server to
_one_ of the referenced source code packages.

We are no longer doing this (we are maintaining a separate notion of
`version` unique to the MCP server), so I don't believe
`package_canonical` is needed anymore. Does anyone see otherwise?

This change removes all mentions in READMEs and usages in code.

scripts/test_publish.sh

@@ -70,7 +70,6 @@ cat > "$PAYLOAD_FILE" << EOF
     "url": "https://github.com/example/test-mcp-server",
     "branch": "main"
   },
-  "package_canonical": "test-mcp-server",
   "registries": [
     {
       "name": "npm",