Require an assigned issue before accepting new-tutorial PRs

[kacperlukawski]

Summary

• New tutorials now require an issue (via a structured template) filed and assigned by a maintainer before a PR is accepted — closes the door on drive-by PRs that promote a third-party tool rather than a Haystack use case.
• A pull_request_target workflow auto-closes PRs that add a new tutorial without referencing an issue (Closes #N) assigned to the PR's author. Edits/fixes to existing tutorials are not gated.
• Added new-tutorial issue template (form), disabled blank issues, added a PR template, and updated CONTRIBUTING.md/README.md to point at the new process.
• Added AGENTS.md so AI coding agents follow the same process and repo conventions (naming convention, index.toml registration, template.ipynb) before generating a PR.

Reference: same policy as deepset-ai/haystack-cookbook#316, adapted to this repo's structure (tutorials/, index.toml, template.ipynb) and scoped so only new-tutorial PRs are gated.

Test plan

• Open a test PR that adds a new tutorial without a linked issue against a fork/branch and confirm the workflow comments + closes it.
• Open a test PR with Closes #<issue> where the issue isn't assigned to the author and confirm it's rejected.
• Open a test PR with Closes #<issue> where the issue is assigned to the author and confirm it passes.
• Open a test PR that only edits an existing tutorial and confirm it is NOT gated.
• Confirm the new issue template renders correctly and blank issues are disabled.

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