Retro #35: falta el runbook de rebase de fork — el próximo fork vuelve a derivar todo el procedimiento #4

Finding (retrospective of instalador #35, severity: medium)

The IntraVox rebase (47 local commits onto a pre-rewrite-published upstream) was improvised end to end: which conflict shapes take which side, when to regenerate (l10n source strings LAST, after every source settle), the reconciliation-commit pattern, and the four-gate verification recipe. Parent #27 still lists two more fork rebases (eurooffice, desktop_workspace). Every lesson lives in one agent session and nowhere else.

Fix

docs/fork-rebase.md here (org level — it serves every fork app), pointed to by each fork repo's AGENTS.md. Contents: the pre-rewrite-upstream twin (a local commit duplicated under another SHA upstream → skip), per-path merge policy (bundles → theirs; ratchets/manifests → regenerate at the end), the reconciliation commit, and the verification recipe (unit suite + phpstan + prebuild chain + zero-deletions diff against the pre-rebase backup).