ci: Add default-branch environment for PAT

[FrederikBolding]

Add default-branch environment for PAT scoping.

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Note

Low Risk
Workflow-only change that just adds an environment gate; main risk is misconfigured environment protections causing the update PR automation to fail.

Overview
The update-pull-request GitHub Actions workflow now runs its PR-update jobs (reacting to the triggering comment, preparing deps, deduping yarn.lock, building, regenerating LavaMoat policies, updating examples, and committing results) under the default-branch environment.

This scopes use of PULL_REQUEST_UPDATE_TOKEN/PAT-backed steps to that environment without changing the workflow logic itself.

^{Written by Cursor Bugbot for commit 5f81f64. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (7689d3a) to head (5f81f64).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3850   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         429      429           
  Lines       12417    12417           
  Branches     1928     1928           
=======================================
  Hits        12242    12242           
  Misses        175      175           

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