fix(ci): stop helm release PRs from piling up and going stale

[rusackas]

SUMMARY

The "Helm: release charts" workflow cuts a new helm-publish-<sha> branch from gh-pages on every helm-touching push to master, and opens a fresh PR each time. Since these PRs are merged manually (by design), they accumulate — and the moment gh-pages moves, the older ones conflict on index.yaml (adjacent version entries plus the generated: timestamp) and have to be hunted down and closed as superseded. Of the last 15 of these PRs, 10 were closed unmerged that way (e.g. #42207, #41692, #41501, #41384, #41345, #41291, #41134).

This changes the workflow to use a single fixed helm-publish branch, force-recreated from the tip of gh-pages on every run:

• There is exactly one evergreen release PR at any time. Each run refreshes it in place instead of spawning a sibling.
• Since chart-releaser rebuilds index.yaml from all published GitHub releases, the branch always carries every chart released since the last merge (this is the same consolidation behavior we already saw in Helm chart release for helm-publish-de390f2 #41167), so nothing is lost when runs happen back-to-back.
• The branch is always freshly cut from origin/gh-pages, so the PR can never go stale or conflict.
• The PR-open step now reuses an existing open PR for the branch instead of failing with a 422.
• A concurrency group serializes runs so two quick pushes can't race on force-pushing the shared branch mid-release.

The manual review-and-merge step is intentionally unchanged.

Not addressed here (not fixable in-repo): the Netlify docs-preview checks fail on these gh-pages-based PRs because that branch has no docs site to build. That's a Netlify site setting (limit deploy previews to PRs targeting master) — worth flipping in the Netlify UI, but it's cosmetic noise, not what kills the PRs.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A (CI workflow change)

TESTING INSTRUCTIONS

1. Merge a change under helm/ (or workflow_dispatch the workflow).
2. Confirm a helm-publish branch is force-created from gh-pages and a single "Helm chart release" PR opens against gh-pages.
3. Trigger the workflow again before merging that PR; confirm the same PR is refreshed (no new PR, run logs "Reusing existing pull request").
4. Merge the PR; the next run recreates the branch and opens a new PR.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

🤖 Generated with Claude Code

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