feat(ui): manage nullclaw-chat-ui from nullclaw CLI

[jd316]

Summary

• add a new top-level nullclaw ui command with install, run, update, and path subcommands
• add src/ui.zig to manage GitHub-release install/update and launch the UI through Node.js
• auto-install UI on first nullclaw ui run and persist active version under ~/.nullclaw/ui
• update CLI help text and README command docs for the new UI workflow

Validation

• zig test src/ui.zig
• zig test src/main.zig --test-filter "parse known commands"
• zig build -Doptimize=ReleaseSmall
• ./zig-out/bin/nullclaw --help
• ./zig-out/bin/nullclaw ui path
• ./zig-out/bin/nullclaw ui update --check (expected NoReleaseFound until a nullclaw-chat-ui release is published)

Known existing issue

• zig build test --summary all still fails on an unrelated pre-existing flaky segfault in session.test.concurrent processMessage different keys — no crash.

[jd316]

Hi @DonPrus
Blocking dependency: publish a nullclaw-chat-ui release first (merge nullclaw/nullclaw-chat-ui#6, then push a v* tag in nullclaw-chat-ui). Until that release exists, nullclaw ui update --check returning NoReleaseFound is expected behavior. 🙏

[DonPrus]

I’d move this functionality to nullhub.

nullhub is the layer that connects ecosystem components and bootstraps them together, so this would keep nullclaw and its UI from becoming tightly coupled.

Repository-based installation is already implemented there, and adding build support would be a great next step.