feat: automated Release Announcement draft maintained by GitHub Copilot

[Copilot]

Adds ReleaseAnnouncement.md — a living draft of the end-user release announcement, separate from ChangeLog, kept up to date automatically by GitHub Copilot as PRs merge to main.

New files

• ReleaseAnnouncement.md — working draft; flat bullet list in the conversational style of real Jamulus Discussion announcements; includes Windows virus-checker REMINDER
• .github/release-announcement-template.md — pristine blank template; replaces the draft after each full release tag
• .github/prompts/release-announcement.prompt.yml — AI prompt with verbatim real announcement examples and authoring rules; maxCompletionTokens: 16384 / temperature: 0.1 (actions/ai-inference default of 200 tokens truncates mid-document)

Workflow: .github/workflows/update-release-announcement.yml

• pull_request_target: closed → main — updates draft on every merged PR; workflow + prompt always sourced from main; PR body written to $RUNNER_TEMP via env: before AI sees it (prevents YAML injection); CHANGELOG: SKIP skips update
• push: branches: autobuild** — updates draft on the autobuild branch so developers can preview announcement impact before merge; runs on any repo (not just jamulussoftware) so fork testing works; loop prevention via github.actor != 'github-actions[bot]'
• push: tags: r* + reset-after-release job — resets to template only on clean version tags (r3_12_0); pre-release tags (beta, rc) leave the draft intact to accumulate through the cycle
• Non-empty guard before overwriting the file — protects against an empty AI response wiping the draft

Supporting changes

• autobuild.yml — ReleaseAnnouncement.md, template, and prompts/** added to paths-ignore so bot commits don't trigger builds
• CONTRIBUTING.md — documents that CHANGELOG: feeds both ChangeLog and ReleaseAnnouncement.md; CHANGELOG: SKIP skips both
• .github/pull_request_template.md — extends CHANGELOG comment to reference Release Announcement

CHANGELOG: SKIP

Context: Fixes an issue?

Implements the approach discussed in jamulussoftware#3443 (comment) and https://github.com/orgs/jamulussoftware/discussions/3626.

Does this change need documentation? What needs to be documented and how?

The release process checklist at https://jamulus.io/contribute/Release-Process could reference ReleaseAnnouncement.md as the draft to polish and publish, replacing the manual "Draft an announcement" step.

Status of this Pull Request

Working implementation. Requires GitHub Models (models: read permission) to be enabled on the repository for actions/ai-inference@v1.

What is missing until this pull request can be merged?

Confirm GitHub Models access is available for this repo.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

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