ci: use a TODO placeholder title for model-regeneration PRs (#901)

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).