refactor: drop the URL-based error classifier and hint lookups

Errors are named by the service call that failed: Classify,
NotFoundError and ConflictError at the call site. Remove the endpoint
parser that guessed the resource from the request path, and the
follow-up reads the explainer made at error time (repository state,
issue-vs-pull-request, the existing pull request's URL) together with
the credentialed transport each API error carried for them.

Calls that the endpoint parser covered and the services did not now
classify themselves: issues, pull requests, reviews, comments,
releases, branches, wiki pages, repositories, organizations and SSH
keys. Unclassified failures fall back to the generic
"not found (HTTP 404 GET ...)" text.

Signed-off-by: bircni <bircni@icloud.com>

internal/transport/api.go

@@ -55,7 +55,7 @@ func (t captureTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	}
 	data, _ := io.ReadAll(io.LimitReader(resp.Body, maxErrorBody))
 	failure := newAPIError(resp.StatusCode, req.Method, req.URL.Path, t.token, data)
-	failure.bind(req.URL, t.base)
+	failure.bind(req.URL)
 	holder.Store(failure)
 	resp.Body = struct {
 		io.Reader