docs: establish AltZen downstream fork workflow

[tetranow]

Summary

• Document AltZen as a downstream fork of Zen Browser using dev as the stable branch.
• Add a guarded PowerShell helper for creating upstream-sync branches.
• Define preservation, sync, feature branch, PR, conflict, and release-build rules.

Notes

The public dev branch has been reset to the clean Zen base commit d76fa13eddf127eaea33b624e82981086987b138.

The previous committed AltZen state is preserved on archive/altzen-opus-1.0.32.

Local uncommitted source work was also snapshotted in the session temp archive before the reset operation.