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.

internal/model/model.go

@@ -122,7 +122,6 @@ type Server struct {
 // ServerDetail represents detailed server information as defined in the spec
 type ServerDetail struct {
 	Server           `json:",inline" bson:",inline"`
-	PackageCanonical string    `json:"package_canonical,omitempty" bson:"package_canonical,omitempty"`
 	Packages         []Package `json:"packages,omitempty" bson:"packages,omitempty"`
 	Remotes          []Remote  `json:"remotes,omitempty" bson:"remotes,omitempty"`
 }