feat: automated Release Announcement draft maintained by GitHub Copilot

[pljones]

Short description of changes

Adds ReleaseAnnouncement.md — a working draft of the end-user release announcement (separate from ChangeLog) that GitHub Copilot keeps up to date automatically, matching the conversational bullet-point style used in real Jamulus Discussion posts.

New files

• ReleaseAnnouncement.md — live draft; flat bullet list; includes Windows virus-checker REMINDER and pointer to tools/get_release_contributors.py
• .github/release-announcement-template.md — pristine blank template; copied over the draft after each full release
• .github/prompts/release-announcement.prompt.yml — AI prompt with verbatim real Jamulus announcement examples; modelParameters.maxCompletionTokens: 16384 / temperature: 0.1 (action default of 200 tokens would truncate 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 passing as file_input (prevents YAML injection); CHANGELOG: SKIP in PR body skips update
• push: branches: autobuild** — updates draft on the autobuild branch itself so developers can preview how their change would read in the announcement before merging; loop prevention via github.actor != 'github-actions[bot]'; autobuild.yml already has ReleaseAnnouncement.md in paths-ignore
• push: tags: r* + reset-after-release job — resets to template only on clean version tags (r3_12_0); pre-release tags (r3_12_0beta1, r3_12_0rc1) intentionally leave the draft intact so it accumulates through the beta/RC cycle
• Non-empty guard before overwriting the file (protects against empty AI response wiping the draft)

autobuild.yml — ReleaseAnnouncement.md, .github/release-announcement-template.md, .github/prompts/** added to paths-ignore so bot commits don't trigger builds.

CONTRIBUTING.md / .github/pull_request_template.md — note that CHANGELOG: feeds both ChangeLog and ReleaseAnnouncement.md; CHANGELOG: SKIP skips both.

CHANGELOG: SKIP

Context: Fixes an issue?

Implements the approach discussed in #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 at the RC stage.

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

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