Use a distinct head branch for main-to-staging syncs with branch-isolated previews

A PR that syncs a release branch back into staging may need a distinct Git head branch when preview provisioning binds a database branch to the Git branch name. Create that head at the exact release commit, confirm its diff against staging, and rerun current-head CI and preview after any base advance. This applies to branch-isolated preview systems with provider branch identity; an HTTP 409 alone does not prove the provider's specific uniqueness rule.

Evidence/example — 2026-09-23: In e3-solutions/theft, a direct main-to-staging sync PR (#991) failed its preview step when Supabase branch creation returned HTTP 409 with PREVIEW_GIT_BRANCH=main (run 35884699039). A replacement sync PR (#992) began at the same main commit on a new Git branch. After staging advanced, its branch was refreshed; the diff remained only the dashboard files, current-head CI and preview passed (run 35886299960, run 35886238029), and the PR merged (PR #992). The observed workaround succeeded here; the exact Supabase constraint behind 409 was not read back from a provider error body.