ci: add automated Decky plugin store update workflow

[Cliffback]

Summary

Adds a GitHub Actions workflow that automatically opens a draft PR against SteamDeckHomebrew/decky-plugin-database whenever a new release is created, updating the LetMeReShade submodule pointer to the latest release tag.

How it works

1. Trigger: Runs automatically after the Release Please workflow completes successfully, or manually via workflow_dispatch with a tag name input.
2. Fork & sync: Forks (or reuses) the decky-plugin-database repo under the PAT owner's account and syncs it with upstream.
3. Update submodule: Checks out the release tag in the plugins/LetMeReShade submodule and commits the change.
4. Open draft PR: Creates a draft PR against upstream with the plugin update template pre-filled (backend answers filled in, checklist boxes left unchecked).

Setup steps required

1. One-time: Plugin transfer PR

The Decky store currently points to itsOwen/LetMeReShade. Before this workflow can function, you must submit a plugin transfer PR to decky-plugin-database to change the submodule URL to Cliffback/LetMeReShade.

Use the plugin_transfer.md PR template. The itsowen README already acknowledges this fork as the maintained version.

2. Create a DECKY_STORE_PAT secret

Create a GitHub Personal Access Token (classic) with:

• repo scope (full control of private repositories — needed to fork and push to the database repo)
• workflow scope (if the database repo has any workflow restrictions)

Then add it as a repository secret named DECKY_STORE_PAT:

1. Go to Settings > Secrets and variables > Actions
2. Click New repository secret
3. Name: DECKY_STORE_PAT
4. Value: the PAT you created

3. After each release

The workflow runs automatically. After it opens a draft PR on decky-plugin-database, you still need to manually:

1. Check the boxes in the PR template (authorship, licensing, SteamOS testing, etc.)
2. Test the plugin on SteamOS Stable/Beta and confirm it works
3. Get a third party to test and leave a comment on the PR
4. Optionally test 2 other plugin PRs and link your feedback (improves priority)
5. Mark the PR as ready for review (remove draft status)

Files changed

• .github/workflows/decky-store-update.yml — new workflow