Collect scriv changelog fragments at release and backfill CHANGELOG.md

[blarghmatey]

What are the relevant tickets?

N/A

Description (What does it do?)

This repo has no changelog, and the release job's commit is only a version bump. This adds scriv fragments that are collected into CHANGELOG.md when a release is cut.

• uv run scriv create writes a fragment under changelog.d/ (Keep a Changelog categories). The README has a short section on it.
• The release image job runs bump-my-version bump --no-commit in the ol-concourse-dsl image (uv since Add a digest-pinned uv to the ol-concourse-dsl image ol-concourse#111, no scriv), then the release resource commits with git add -u. A bump-my-version pre_commit_hooks entry runs bin/collect-changelog through uv run --frozen --only-group release. bump-my-version runs pre-commit hooks under --no-commit too, after pyproject.toml has the new version, which is where scriv reads it from.
• bin/collect-changelog wraps scriv collect so an empty changelog.d/ is a no-op. scriv collect exits 2 with no fragments, which would fail a Renovate-only release.
• scriv is in its own release dependency group (included in dev) so the release task installs only scriv from the lockfile.
• CHANGELOG.md is backfilled for 2026.7.22.1 through 2026.9.30.1 from the commits and PRs between tags. It has to be tracked before the first release because git add -u doesn't stage new files.

How can this be tested?

I reproduced the release task in the mitodl/ol-concourse-dsl:latest image the job uses, as published after ol-concourse#111 (bump-my-version 1.5.1, uv 0.12.15, no scriv): committed the tree, ran bump-my-version bump --new-version 2026.10.1.1 --no-commit --allow-dirty, then git add -u.

• With the fragment in this PR, the staged changes were CHANGELOG.md (new 2026.10.1.1 entry), the deleted fragment, pyproject.toml, and uv.lock.
• With changelog.d/ empty, the bump succeeded and only pyproject.toml and uv.lock changed.

It hasn't run in Concourse. The next release cut is the real test.

Additional Context

• Hotfixes will get the wrong changelog entry. bump_version_task runs on main HEAD, and _create_hotfix stashes that tree and pops it onto production plus the cherry-picked commit. So the hotfix entry lists every unreleased fragment on main, and because the fragment deletions don't apply on the hotfix branch, those fragments stay on main and are listed again in the next normal release. Hotfixes are rare enough that I'd rather hand-edit the entry than fix this in the release resource now.
• A fragment merged between the job's get of main and the cut is attributed to the next release. The version bump already has this race.
• Nothing enforces a fragment per PR yet. If we want that, a CI check with a skip label for Renovate would be the follow-up.
• Please sanity-check the categorization in the backfill; it's my read of each PR.

[Copilot]

Copilot review overview

🟡 Changes recommended

The release-critical collection branches need automated coverage, alongside the smaller configuration and wording corrections.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Use supported Scriv setting name changelog · New
• Correct changelog claim that every PR adds a fragment · New

What changed in this PR

Adds Scriv-based changelog fragments and release-time collection, with historical releases backfilled.

Changes:

• Configures Scriv and release dependencies.
• Adds a release hook that safely handles empty fragment directories.
• Documents the workflow and backfills release history.

File	Description
README.md	Documents changelog fragments.
CHANGELOG.md	Backfills historical releases.
bin/​collect-changelog	Collects fragments during releases.
changelog.d/​.gitkeep	Preserves the fragment directory.
changelog.d/​20260930_101301_blarghmatey_scriv_changelog.md	Describes this feature.
pyproject.toml	Configures Scriv and the release hook.
uv.lock	Locks Scriv and dependencies.

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[blarghmatey]

Added tests/test_collect_changelog.py in 3254426 for the review's coverage note. It runs bin/collect-changelog against the repo's real scriv config with and without fragments.