fix deploy: update zig version in Dockerfile, target baseline x86_64
- Zig 0.16.0-dev.3059 -> 0.16.0-dev.3070 to match zat requirement
- Explicitly target x86_64-linux-gnu with baseline CPU to avoid SIGILL
on Fly VMs that lack extended instruction sets
- Bump bot threshold from 50 to 500 posts/24h (50 was too aggressive)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>