docs(fork): add rebuild script + fork maintenance guide

Why: a one-command way to rebuild 'our Orca' (native arm64), fold in official
updates (--sync), and produce a shareable installer (--dmg); plus the model for
running ours alongside the official app without clobbering.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FNy4vrv3mvyjpPnhJMZr82

Orca Mobile iOS 0.0.15