infra(gpu-box): commit GPU-Box compose to repo + Phase 2e docs

The GPU-Box stack has been carrying real production workload since
Phase 2c (monitoring) but only existed as a /srv/mana/docker-compose.gpu-box.yml
on the box itself. If the WSL filesystem dies, none of it is
reproducible. Bring the file into infrastructure/ as the source of
truth (live file on the box must be kept synchronous; manual rsync
for now since there's no CD into the GPU box).

Plus:
- infrastructure/.env.gpu-box.example as the secrets template
- infrastructure/README.md describing what runs there + how the
  Cloudflare-tunnel ingress is API-managed (not config.yml)
- .gitignore for the live infrastructure/.env.gpu-box copy
- MAC_MINI_SERVER.md status-page section now points at the GPU-Box
  setup instead of the long-stopped Mini container
- PLAN_OPTION_C.md: Phase 2e row + GPU-Box service tree update

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -125,6 +125,9 @@ yarn.lock
 .env.macmini
 ssh-key-command.txt
 
+# GPU-Box deployment — live .env, copy of infrastructure/.env.gpu-box.example
+infrastructure/.env.gpu-box
+
 # Python
 .venv/
 __pycache__/