Add nullclaw instance admin api and upgrade to zig 0.16

[DonPrus]

What changed

• moved the nullclaw instance admin surface into nullhub so instance runtime/config/model/cron management can be handled centrally
• added instance-scoped config value reads, models listing, runtime status fields, and cron CRUD/control endpoints for nullclaw instances
• upgraded nullhub to Zig 0.16 and added compatibility shims for fs/process/net/time/sync APIs
• updated build/test/release configuration to use Zig 0.16 and kept release/test builds green

Why

The admin REST API from nullclaw#770 is a better fit in nullhub, where instance orchestration already lives. This keeps nullclaw focused on the runtime itself and lets nullhub expose a single control plane for managed instances.

Impact

• nullhub can now serve the missing pieces needed to replace per-instance admin endpoints for managed nullclaw installs
• runtime responses include richer per-instance details such as pid, uptime, restart count, and port when available
• cron operations are proxied through nullhub into the target nullclaw instance home
• the repository now builds and tests on Zig 0.16

Validation

• zig build -Doptimize=ReleaseSmall
• zig build test --summary all