Restore url.PathEscape on user-supplied path segments in url.JoinPath calls

url.JoinPath does not percent-encode slashes or reserved characters in
its arguments, so user-supplied values (group IDs, slugs, team IDs) must
be wrapped with url.PathEscape to prevent path traversal. Update cursor
rule and contrib guide to codify this as a mandatory practice.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/accessreview/drivers/heroku.go

@@ -72,7 +72,7 @@ type herokuTeamMember struct {
 func (d *HerokuDriver) ListAccounts(ctx context.Context) ([]AccountRecord, error) {
 	var records []AccountRecord
 
-	endpoint, err := url.JoinPath("https://api.heroku.com", "teams", d.teamID, "members")
+	endpoint, err := url.JoinPath("https://api.heroku.com", "teams", url.PathEscape(d.teamID), "members")
 	if err != nil {
 		return nil, fmt.Errorf("cannot build heroku members URL: %w", err)
 	}