Migrate PR quality checks from Node.js to Go (#6055)

* automate PR review: rewrite quality checks in Go and add diff validation

- Replace check-quality.js with Go implementation that adds:
  open source license detection, repo maturity (5+ months),
  CI/CD presence check, and README existence verification
- Add new check-pr-diff tool that validates README changes:
  single item per PR, link consistency with PR body,
  description format, non-promotional language, and
  category minimum size (3 items)
- Update pr-quality-check workflow to run both Go tools
  with separate quality and diff reporting sections
- Improve CONTRIBUTING.md with new "What is checked automatically"
  section documenting all blocking and non-blocking CI checks,
  plus what still requires manual maintainer review
- Improve PR template with clearer structure separating
  CI-validated requirements from manual review items

https://claude.ai/code/session_01JcdbDdtERhAXFFZmu3pCVY

* add actionable fix instructions to automated PR review comments

Each failed check now includes a "How to fix" section with specific
commands and steps the contributor needs to take, e.g.:
- missing go.mod → shows exact `go mod init` + `go mod tidy` commands
- missing semver release → shows `git tag v1.0.0` command and GitHub UI link
- low Go Report Card grade → shows `gofmt`, `go vet` commands
- missing PR body links → shows the exact text to paste
- description without period → shows the corrected entry line
- link mismatch → shows both URLs and which to update

https://claude.ai/code/session_01JcdbDdtERhAXFFZmu3pCVY

* add auto-merge when all PR quality checks pass

Adds an auto-merge job that enables GitHub's native auto-merge (squash)
after the quality and diff checks succeed. GitHub will only execute the
merge once ALL required branch protection status checks are green
(including the tests workflow).

Requires "Allow auto-merge" to be enabled in repo settings.

https://claude.ai/code/session_01JcdbDdtERhAXFFZmu3pCVY

* add workflow to re-run quality checks on all open PRs

Creates a manual workflow_dispatch workflow that iterates through all
open PRs and closes/reopens each one to re-trigger the quality check
pipeline. Includes a 2s delay between PRs to respect API rate limits.

Usage: Actions tab → "Re-check all open PRs" → Run workflow

https://claude.ai/code/session_01JcdbDdtERhAXFFZmu3pCVY

* skip quality checks for PRs that don't modify README.md

The quality checks (forge link, pkg.go.dev, Go Report Card, etc.) only
apply to package addition/removal PRs. Internal PRs (workflow changes,
docs, scripts) were failing because they naturally don't have those
fields in the PR body.

Now the workflow first detects if README.md is in the changed files:
- If yes: runs full quality + diff checks as before
- If no: posts a "Skipped" notice and passes cleanly

Fixes false failures like PR #6055.

https://claude.ai/code/session_01JcdbDdtERhAXFFZmu3pCVY

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Co-authored-by: Claude <[email protected]>