ci: add Packit propose_downstream automation for Fedora dist-git updates

[samcday]

Motivation

• Enable upstream-side automation so that new releases can automatically open Fedora dist-git update PRs using Packit's propose_downstream job.

Description

• Add a propose_downstream job to the upstream .packit.yaml configured with trigger: release and dist_git_branches: [fedora-all] so release events open dist-git PRs.
• Keep existing COPR jobs and other Packit configuration unchanged and preserve specfile_path and files_to_sync entries.
• Commit created with message ci: add packit propose_downstream job updating .packit.yaml.

Testing

• Ran git diff --check which reported no issues and the change was committed successfully.
• Attempted yamllint .packit.yaml but yamllint is not available in this environment, so YAML linting was not performed.
• Verified the resulting .packit.yaml contains the new propose_downstream job at lines 40–44 via git show and nl output.

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Codex Task