Extract the fat inline bash steps from upstream-release-docs.yml into scripts/

.github/workflows/upstream-release-docs.yml is 1667 lines across 41 steps, including 960 lines of inline bash. The size is concentrated in four steps:

Two reasons to extract the big ones:

• Nothing lints the inline bash. There is no shellcheck or actionlint in CI, so checking a change today means hand-extracting steps[].run from the YAML, stubbing ${{ }}, and piping it to shellcheck.
• scripts/upstream-release/ already holds five .mjs files and this workflow already invokes one, so a script there inherits Prettier and ESLint from existing CI with no new tooling.

Augment PR body is the best first candidate: its only I/O is roughly 25 env vars in and one gh pr edit --body-file out, and the PR body is the artifact reviewers actually read.

Scope

In:

• Augment PR body to scripts/upstream-release/render-pr-body.mjs
• Assign reviewers and Resolve release owner and contributors as separate follow-ups if the first goes cleanly

Out:

• The two Claude step prompts (88 lines). Their content is almost entirely interpolated run context, and the durable instructions already live in the upstream-release-docs skill.
• Capture skill signal files, mostly file reads and GITHUB_OUTPUT heredocs.
• The 37 short steps, which belong inline.

Sequencing

After #1111 has run live on two or three release PRs. That change is unvalidated, and stacking a refactor on it means a live failure has two candidate causes instead of one.

#1111 and this refactor touch the same steps, so if #1111 needs iteration, extract Augment PR body first (#1111 only added two table rows and two sections to it) and leave the reviewer steps until after validation.

Verification

Behavior-preserving: render the extracted script against the env of a real past run and diff the output against that run's actual PR body.

Read .github/workflows/upstream-release-docs.yml, the Augment PR body step, and the existing files in scripts/upstream-release/. Use the environment from a real past run to compare the extracted script's output with that run's actual PR body; done means behavior is preserved and the script is covered by the existing formatting and linting setup.