Tooling: fork-follow autopatcher (upstream + patch-replay, staging gate, rollback) - contribution proposal

Context

Part of the fork maintenance series around #48638. Related: #49565.

Proposal

A reproducible "fork-follow" autopatcher used to keep a locally built opencode binary in sync with upstream dev + the fork's patch series:

• upstream + patch-replay build (patch = full branch diff, replayed with --3way and guarded apply)
• composite skip key (upstream SHA + patch state + patch SHA) for cheap no-op checks
• patch-cache self-healing (bare, blob-filtered, single-branch clone)
• schema gate: an unpatched build is never auto-activated over a DB carrying patch-only migrations
• live-session gate: builds are staged and activated on a later launch; no hot-swap under running sessions
• dry-run mode, bounded log rotation, pre-migration DB backup, rollback tool

Currently machine-local (shell scripts). If maintainers see value, this could be contributed as a small standalone PR (scripts + fixture-based lifecycle tests). Otherwise this issue can be closed as informational.

Linked: PR #48638 · tracker #49565 · sibling tracking issues listed in a follow-up comment.

Start by reviewing the context in #48638 and the related tracker #49565, then examine the existing machine-local shell scripts referenced by the proposal. The work would be complete when a standalone contribution covers the stated replay, staging, rollback, and gating behavior with fixture-based lifecycle tests; maintainers must first confirm that this proposal is wanted.