Add changelog-release-pr composite action

[yonib05]

Description

Adds a changelog-release-pr/ composite action that turns GitHub Releases into structured changelog markdown files and opens a PR into strands-agents/harness-sdk (where the changelog page lives). This is the automation half of the Strands changelog feature; the site/page half is a separate harness-sdk PR.

Deterministic, no LLM. Pipeline: list/get release(s) → parse the auto-generated "What's Changed" body into structured entries → enrich each from its linked PR (area-* labels → areas, breaking change label, merge-commit SHA, author) → render a markdown file matching the harness-sdk changelog content-collection schema (preserving any human-written highlights/body on re-sync) → open a PR via peter-evans/create-pull-request.

Built to match devtools house style: plain dependency-free .cjs modules invoked through actions/github-script (like strands-command/scripts/javascript/process-input.cjs), authenticated with a single github-token input (mirrors strands-finalize; no AWS). Handles the historical wrinkle that older harness releases link PRs on the old strands-agents/sdk-python repo — enrichment and links use each PR's own repo.

Type of Change

New feature

Testing

• 38 unit tests via Node's built-in runner: cd changelog-release-pr/scripts && node --test (all pass). Cover tag→sdk/language mapping, release-body parsing (incl. CRLF, breaking markers, omitted-list, format-drift guard), PR enrichment + graceful degradation, YAML-safe markdown rendering (quoting reserved words / significant chars), highlights/body preservation on re-sync, and release selection (single/backfill).
• End-to-end validated against the real harness-sdk Zod schema: generated files for a harness-python, harness-typescript, evals, and pre-monorepo bare-v release, dropped into the harness-sdk content collection and run through astro sync — synced with zero schema errors.

Notes for reviewers

• Consumed by changelog-sync.yml workflows in harness-sdk (same-repo, github.token) and evals (cross-repo PR into harness-sdk, needs a token with pull-requests:write on harness-sdk — see that PR).
• Backfill of all ~107 historical releases is run via workflow_dispatch after this merges.