[repo]: migrate plugin ZIPs to GitHub Releases

[sethwv]

Moves all plugin ZIP distribution from git-tracked files on the releases branch to GitHub Release assets, and renames zips/ to metadata/ on the releases branch.

What changed

• ZIPs are now published as GitHub Release assets (tags like plugin-1.0.0 and plugin-1.0.0-latest) instead of committed files
• zips/ directory on the releases branch renamed to metadata/ (manifests and READMEs only)
• All scripts updated to read version/download data from manifests and release tags instead of the filesystem
• New run-migrations.yml workflow handles the one-time migration of existing ZIPs to releases and the zips/ rename, with dry-run support and a REPO_VER guard
• README.md and CONTRIBUTING.md updated to reflect the new URLs and process
• Bumped all GitHub Actions to latest major versions: checkout@v6, cache@v5, upload-artifact@v7, download-artifact@v8, create-github-app-token@v3; renamed deprecated app-id input to client-id

Migration notes

Run the migrations workflow before the next publish. Migration 001 moves existing ZIPs to GitHub Releases; migration 002 does the zips/ rename. The publish workflow will not proceed until REPO_VER reflects the current schema.

[dispatcharr-plugins-bot]

This PR is currently a draft. Plugin validation will run once the PR is marked ready for review.