ci(security): push with default GITHUB_TOKEN, not App token (#361)

The fix-dependabot-alerts workflow was using the GitHub App token for
git push, which required granting the App `Contents: write` at the
installation level. That permission wasn't (and didn't need to be)
granted, causing the scheduled run to 403 at `git push` with
'Permission to microsoft/TypeChat.git denied to typeagent-bot[bot]'.

Match the TypeAgent workflow pattern instead: keep
`persist-credentials: false` on checkout (so the token isn't reachable
from untrusted `npm` scripts during the verify phase), but at the very
end of the job re-inject the workflow's own GITHUB_TOKEN — already
scoped to `contents: write` via the workflow-level `permissions:`
block — for the git push.

The App token is now used only where it must be:
- `gh api dependabot/alerts` (the default GITHUB_TOKEN can't reach
  this endpoint)
- `gh pr create` / labels / closing superseded PRs (so the PR
  identity is the bot, not github-actions)

Verified end-to-end: a manual workflow_dispatch run against a temp
branch passed git push and opened #360.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: TalZaccai

.github/workflows/fix-dependabot-alerts.yml

@@ -162,7 +162,17 @@ jobs:
       - name: Create pull request
         if: ${{ steps.fix.outputs.changes == 'true' && steps.build.outputs.build_ok == 'true' }}
         env:
+          # GH_TOKEN is the App token — used by the ``gh`` CLI for
+          # ``gh pr create`` / labelling / closing superseded PRs so the
+          # PR appears under the bot's identity.
           GH_TOKEN: ${{ steps.app-token-pr.outputs.token }}
+          # GIT_PUSH_TOKEN is the workflow's default GITHUB_TOKEN, scoped
+          # via the workflow-level ``permissions: contents: write`` block.
+          # We use it only at the very end, after all untrusted ``npm``
+          # scripts have finished running, to avoid persisting any push
+          # credential in .git/config (where dependency build scripts
+          # could read it).
+          GIT_PUSH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           BRANCH="automated/fix-dependabot-alerts-$(date +%Y%m%d)-${{ github.run_number }}"
           git config user.name "github-actions[bot]"
@@ -188,10 +198,12 @@ jobs:
           Unfixable: ${{ steps.fix.outputs.unfixable_count }} package(s)
 
           Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>"
-          # Push using the App token explicitly — we disabled
-          # ``persist-credentials`` on checkout, so .git/config has no
-          # creds to fall back on.
-          git remote set-url origin "https://x-access-token:${GH_TOKEN}@github.com/${{ github.repository }}.git"
+          # Push using the workflow's default GITHUB_TOKEN (scoped to
+          # contents:write at the workflow level). Configured here, not
+          # via actions/checkout's ``persist-credentials``, so the token
+          # isn't reachable from the npm install / build / test phase
+          # earlier in the job.
+          git remote set-url origin "https://x-access-token:${GIT_PUSH_TOKEN}@github.com/${{ github.repository }}.git"
           git push origin "$BRANCH"
 
           APPLIED="${{ steps.fix.outputs.applied_packages }}"