Pin generated workflows to ubuntu-24.04

[arvida]

Needs Arvid's decision before merge. This is the pinning option from localheroai/localhero-ai#724, prepared so it is ready if you want it. Nothing is broken on Ubuntu 26 today (see below), so merging is a policy choice, not a fix.

Why

GitHub moves ubuntu-latest to the Ubuntu 26 image between October 19 and November 19. The workflow init writes lives in the customer's repository, so if a future image changes the toolchain under it (the Django path installs gettext from apt), we cannot fix the file for them. Pinning ubuntu-24.04 keeps that toolchain fixed for the two years GitHub supports the image.

Trade-off: pinned workflows age out. In about two years every repo created with this version needs a manual bump, or it stops running when GitHub retires the image.

Change

• src/utils/github.ts: the generated job uses runs-on: ubuntu-24.04 via a RUNNER_IMAGE constant.
• Test in tests/utils/github.test.ts (written first, failed on ubuntu-latest).
• CHANGELOG entry under Unreleased.

Our own CI (.github/workflows/test.yml) stays on ubuntu-latest: breakage there is visible to us and cheap to fix. Existing customer workflows are unchanged either way.

Evidence from Ubuntu 26

django-trial-rig PR localheroai/django-trial-rig#12, run 37839209218, runs-on: ubuntu-26.04 (image ubuntu-26.04 version 20260927.149.1, Ubuntu 26.04.1 LTS), CLI 0.0.80:

• sudo apt-get install -y -qq gettext installed gettext 0.23.2-1 without an apt-get update
• actions/setup-python@v5 with 3.12: Successfully set up CPython (3.12.14), pip 26.2.1
• makemessages --keep-pot -l sv -l ru -l pl: processed all three locales
• Localhero CLI: Updated 1 keys in 3 languages, commit pushed to the PR branch
• uv probe job on the same image: setup-uv@v5 installed uv 0.12.24, uv pip install plus uv run python manage.py makemessages updated the .pot and all three .po files

Verified

• npm test: 82 suites, 1496 tests pass
• npx tsc --noEmit, npx eslint 'src/**/*.ts': clean
• Codex review of the diff: no findings

Companion server change for the setup PR: localheroai/localhero-ai#875. If this is approved, the workflow examples on the docs pages and blog posts still say ubuntu-latest and need a follow-up.