chore(ci): split release.yml into build/publish/release-notes/notify jobs

[mscolnick]

Summary

Restructures the release workflow into four jobs with an artifact handoff so the id-token: write permission is scoped to a job that does nothing but npm publish. This resolves the supply-chain audit's oidc-publish-fused finding (OIDC publish should not share a job with arbitrary install/build/test code).

Jobs

• build (no id-token): checkout, setup pnpm + Node (no registry-url), pnpm install --frozen-lockfile, test, build, validate dist/index.js + dist/index.d.ts, then upload dist/, package.json, README.md, LICENSE as the package artifact (1d retention, fail if empty).
• publish (needs: build, id-token: write + contents: read): setup Node with registry-url, download the package artifact, run npm publish --ignore-scripts with NODE_AUTH_TOKEN="" to enable OIDC. No source checkout, no install — nothing else runs in this token's blast radius.
• release-notes (needs: publish, contents: write): checkout and run npx changelogithub.
• notify (needs: [build, publish, release-notes], if: always()): existing Slack release-notification step, status derived from contains(needs.*.result, 'failure').

Workflow-level permissions dropped to contents: read. All preexisting action SHAs preserved; actions/upload-artifact and actions/download-artifact pinned to v4 SHAs.

Test plan

• Next tag push triggers all four jobs in sequence
• publish job successfully authenticates to npm via OIDC using only the downloaded artifact
• Slack notification fires with correct status

[Copilot]

Pull request overview

Refactors the release GitHub Actions workflow to isolate npm OIDC publishing into a dedicated job, reducing token blast radius and addressing the “OIDC publish fused with build steps” supply-chain finding.

Changes:

• Split the previous single release job into build, publish, release-notes, and notify jobs.
• Added artifact handoff from build → publish so the OIDC-enabled publish job runs without checkout/install/build steps.
• Reduced workflow-level permissions to contents: read, with job-level overrides where needed.

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