Migrate release notes to per-PR fragments #2353

Background

Release notes for the Upcoming release are currently edited directly in docs/release-notes.md, so concurrent PRs touching that file produce review conflicts and rebase churn. We're moving to a fragment-based workflow:

• Each PR with a user-visible change adds a small Markdown fragment under docs/release-notes.d/, named <slug>.<category>.md.
• PR-time CI requires a fragment (or the skip-notes label) and validates its name and contents.
• At release time, the release-notes tool folds fragments into the Upcoming section of docs/release-notes.md, scaffolds the next Upcoming section, and deletes the consumed fragments. PR links are derived from Git history, not manual bookkeeping.

Rollout status

• docs/release-notes: use per-PR fragments #2347 — migrate the current Upcoming notes into fragments and update contributor docs
• Sync repo templates ⚙ #2352 — repo-templates sync delivering the tool, validation workflow, and release-note skill (needs the skip-notes label; the new check applies to the sync PR itself)

Supporting work already merged:

• release-notes: support per-repo fragment checks repo-templates#379 — fragment tool, validation workflow, skill, and docs (source of truth, synced downstream)
• require-file-change: support regex and status filters actions-lib#13 — require-file-change gains path-regex and status inputs used by the new workflow

Follow-ups

• Future repo-templates sync PRs will fail the new "Require release note" check since they never add fragments; repo-templates should auto-label its sync PRs with skip-notes or the workflow should ignore the sync branch
• Require PR submitters to provide a reason for skipping notes actions-lib#7 — require a stated reason when using the skip label

Once #2347 and #2352 land, this can be closed; remaining follow-ups can move to their own issues if still relevant.