Construct orgClient inside github-script to fix module resolution

@actions/github is only resolvable from inside the github-script bundle,
not from main.js. Build the PAT-authed Octokit client in action.yml and
pass it into main as orgClient. Switch the team-membership check to use
orgClient.rest.teams.getMembershipForUserInOrg instead of a raw request
with header override.

slash-commands/action.yml

@@ -99,5 +99,11 @@ runs:
       with:
         github-token: ${{ inputs.bot-token }}
         script: |
+          // Construct a second Octokit client authed with the PAT for
+          // org team-membership checks. This has to happen here, not in
+          // main.js, because @actions/github is only resolvable from
+          // inside the github-script bundle.
+          const { getOctokit } = require('@actions/github');
+          const orgClient = getOctokit(process.env.ORG_TOKEN);
           const main = require(process.env.GITHUB_ACTION_PATH + '/main.js');
-          await main({ github, context, core });
+          await main({ github, context, core, orgClient });