ci: Drop cross-repo model regeneration dispatch to apify-client-python

[vdusek]

Removes the cross-repo dispatch that opened a companion model-regeneration PR in apify-client-python for every spec-changing docs PR, and rewrites the AGENTS.md section describing it.

apify-client-python now pulls the spec on its own nightly schedule instead, so nothing on this side needs to push to it or be merged in lockstep with a client PR.

Why the dispatch goes away:

• The client can only generate models from the published spec, so it can never be genuinely ahead of a deployed docs change - the companion PR was opened against an unpublished bundle it would have to regenerate from anyway.
• Two mechanisms writing the same generated files made them diverge. The dispatch reused one long-lived branch per docs PR and regenerated with the codegen tooling from that stale branch, which silently reverted fixes that had landed on the client's master in the meantime.
• Every such PR opened with a placeholder TODO title that a human had to replace before it could be merged, so unmerged PRs and their stale branches piled up.

Also drops what only the removed jobs needed: the closed pull request trigger, the if: guard skipping lint on close events, and the pull-requests: read permission.

Merge this before apify/apify-client-python#983 - that PR deletes manual_regenerate_models.yaml, which the dispatch removed here still calls.

✍️ Drafted by Claude Code

[apify-service-account]

✅ Preview for this PR (commit 51344194) is ready at https://pr-2835.preview.docs.apify.com (see action run).