Two fixes for intermittent release workflow failures:
- **Use `BASE_BRANCH` instead of `DEFAULT_BRANCH`** for the PR `base` field — `BASE_BRANCH` has a `:-main` fallback, preventing an empty base from causing a 404
- **Replace fixed `sleep 3` with proper polling and retry** — polls for branch readiness (up to 10 attempts) before creating the PR, then retries PR creation (up to 5 attempts) with backoff
Fixes the `Error creating PR (HTTP 404)` seen when Gitea hasn't fully indexed the `next-release` branch by the time the PR creation request fires.
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Reviewed-on: #19
argoyle
e3c17ceaa0