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.

api/openapi.yaml

@@ -333,10 +333,6 @@ components:
         - $ref: '#/components/schemas/Server'
         - type: object
           properties:
-            package_canonical:
-              type: string
-              enum: [npm, docker, pypi, crates]
-              example: "npm"
             packages:
               type: array
               items: