fix(instances): stage native dev-local builds and report live status

[vernonstinebaker]

Summary

• stage dev-local binaries at the canonical ~/.nullhub/bin/<component> path and prefer local zig-out/bin/<component>-native builds when available
• refresh managed dev-local binaries before start so rebuilt local binaries are picked up without manual cache cleanup
• report imported standalone nullclaw/default status from its live gateway health instead of stale manager state, and ignore zombie PIDs on macOS

Why

• the managed-instance regression was caused by NullHub launching an outdated staged dev-local binary, not by generated config
• imported standalone default could also remain falsely running because the old runtime state kept a zombie pid that kill(pid, 0) still treated as alive on macOS
• together these left NullHub showing or starting the wrong runtime even after the fixed local NullClaw binary was available

Validation

• zig build
• zig build test -Dbuild-ui=false -Dembed-ui=false
• live NullHub smoke against http://127.0.0.1:19800 with default, willy-nilly, and instance-1 all reporting healthy
• verified managed dev-local staging now matches the rebuilt local NullClaw binary by sha256

Notes

• this PR intentionally does not patch generated NullClaw config as a runtime workaround
• provider probe timeout tuning remains in feat(providers): /models probe for custom endpoint validation #34

[vernonstinebaker]

This is the focused follow-up for the managed NullClaw startup regression uncovered during validation of #34. Downstream context: NullClaw already contains the daemon nonblocking-accept fix, and the remaining issue for NullHub-managed instances was stale staged binaries. Related downstream PR context: nullclaw/nullclaw#878.

[vernonstinebaker]

This is the focused follow-up for the managed NullClaw startup regression uncovered during validation of #34.

Downstream context: NullClaw main already contains the daemon nonblocking-accept fix, and the remaining issue for NullHub-managed instances was stale staged dev-local binaries.

Related downstream PR context: nullclaw/nullclaw#878.