ci: avoid branch clash in regenerate-models workflow

[vdusek]

Summary

Fixes the branch-clash bug in .github/workflows/manual_regenerate_models.yaml that was flagged as a follow-up in #809. The recent manual run (failing CI) failed at the Commit model changes step with:

fatal: refusing to fetch into branch 'refs/heads/update-models-manual' checked out at '...'

Cause

The local git switch -c "$BRANCH" step pre-created the branch unconditionally. apify/actions/signed-commit@v1.0.0 with create-branch: 'true' then ran git fetch --depth=1 origin "$BRANCH:refs/heads/$BRANCH", which refuses to fetch into a checked-out branch.

Fix

• When the remote branch already exists: fetch + check it out locally, and tell signed-commit create-branch: false.
• When it doesn't exist: stay on the default branch and let signed-commit create the remote branch with create-branch: true.

The create_branch decision is exported from the renamed Set up branch step and consumed by the signed-commit step.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.52%. Comparing base (252e0e4) to head (8e92095).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
+ Coverage   94.50%   94.52%   +0.02%     
==========================================
  Files          48       48              
  Lines        4916     4916              
==========================================
+ Hits         4646     4647       +1     
+ Misses        270      269       -1     

Flag	Coverage Δ
integration	90.54% <ø> (-0.17%)	⬇️
unit	83.03% <ø> (ø)

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[vdusek]

It works now: #813