Add encrypted vault backup commands

[nullwiz]

Summary

• add envault backup init/status/commit/push for backing up encrypted vault files via Git
• keep Git behavior isolated in backup.zig, leaving main.zig as a command dispatcher
• document that backup commits stage only encrypted *.env.age files

Verification

• zig build test
• zig build
• smoke-tested backup init and backup status with a temporary ENVAULT_ROOT and nested github.com/owner/repo/dev.env.age path