Generate release notes from merged pull requests

[TheGreatAxios]

Closes CL-7888.

Summary

Cutting a release required hand-writing a ## [X.Y.Z] section in CHANGELOG.md first, and release.sh hard-failed without one (no ## [$VERSION] section in CHANGELOG.md — rename [Unreleased] first). That is a whole pull request per release whose only job is narrating the previous ones — written after the fact by whoever remembers, and free to drift from what actually merged.

release.sh now asks GitHub for the notes covering every pull request merged since the previous tag, each credited to its author. faremeter/interchange releases this way.

The runtime feature stays

This is not "delete the changelog" — it is "stop writing it by hand". src/changelog/index.ts still parses the file, /changelog still works, the post-upgrade what's-new still works, and CHANGELOG.md still ships in DOC_FILES so offline installs carry their notes. Only the source of the text changes.

Precedence: --notes <file>, then an existing hand-written section, then generated. A release that deserves narration can still have it.

Two things I got wrong first, and verified

The tag does not exist yet when notes are resolved. I flagged this as the likely awkward part on the issue. target_commitish anchors the range end at the commit being released — confirmed against the live API, which returned the full 0.3.24..HEAD PR list for an unborn v0.3.26.

Generated bodies open with their own h2s (## What's Changed, ## New Contributors). parseChangelogText ends a version section at the next line starting with ## , so writing the body in verbatim truncated the entry to its header — /changelog would have rendered an empty release. Caught by round-tripping real generated output through the actual parser, not by reading it. Headings are demoted to h3 on write.

After the fix, that round-trip gives:

top version:       0.3.26
body lines kept:   28
has PR links:      true
has author credit: true
entries:           56 (was 55)
next entry intact: 0.3.24 -> 24 lines

Also

The CHANGELOG.md preamble claimed the file is the hand-maintained source and told you to rename ## [Unreleased] at cut time. Both are now false; it describes generation instead, and marks that everything below predates it.

Note on grouping

GitHub's .github/release.yml groups by pull-request labels, not commit-message type. The Conventional Commits work in #985 makes the generated list read better but does not by itself produce grouped sections. Interchange's notes are ungrouped and read fine, so grouping is optional polish — not added here.

Testing

bash -n scripts/release.sh, changelog parser tests (32 pass), tsc --noEmit, bun run lint. The generate-notes call and the parser round-trip were both exercised against the real repo. The full release.sh run is not exercised — it tags and publishes.

[linear-code]

CL-7888

[TheGreatAxios]

Audit: Approve.

Fit: generated release notes from merged PRs is the right source of truth going forward — it removes hand-written notes drift and matches how GitHub already credits authors. Scope stays tight to the release path.

Fix note: one verified dead branch fixed on this branch (b409727) — in scripts/release.sh write_changelog_section, the first=$(grep -n '^## \\[[0-9]' … | head -1 | cut …) pipeline aborts under set -euo pipefail when zero versioned headers exist, so the if [ -z "$first" ] fallback was unreachable. Appended || true to the pipeline; verified the fallback now appends and the insert-before-first path still works. Same commit also moves the release commit message and release PR title to the chore(release): form so this branch lands consistent with the Conventional Commits rule.