ci: use a TODO placeholder title for model-regeneration PRs

[vdusek]

The auto-opened model-regeneration PR used to default its title to [TODO]: update generated models from apify-docs PR #N. That looked finished enough to merge as-is and tied the title to apify-docs, even though the right title depends on the actual model diff (a routine chore: sync, or a refactor:/feat:/fix: when models meaningfully change).

The PR title is now a bare TODO placeholder. It carries no valid Conventional Commits type, so pr-title-check stays red until a human replaces it. The apify-docs reference and link, plus a > [!IMPORTANT] note asking the assignee to replace both the title and the description, now live in the PR body. The automated regeneration commit keeps its own descriptive chore: message (separate from the PR title).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.54%. Comparing base (8c48a06) to head (2256199).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #901      +/-   ##
==========================================
+ Coverage   94.52%   94.54%   +0.01%     
==========================================
  Files          48       48              
  Lines        5098     5098              
==========================================
+ Hits         4819     4820       +1     
+ Misses        279      278       -1     

Flag	Coverage Δ
integration	92.93% <ø> (+0.31%)	⬆️
unit	83.36% <ø> (ø)

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