fix(installer): skip re-download when release binary is already cached

[vernonstinebaker]

Summary

• downloader.download() was called unconditionally on every install, even when the binary was already present in ~/.nullhub/bin/
• This forced every reinstall (e.g. reconfiguring provider credentials on an existing instance) to pay the full network round-trip cost for a binary that hadn't changed
• Add the same existence check that stageLocalBinary already uses: attempt to open the target path; if it succeeds, the binary is cached and the download is skipped entirely

Validation

• zig build test -Dbuild-ui=false --summary all — all tests pass, 0 leaks
• zig fmt --check src/ — clean
• Manual test: installed nullclaw once (binary downloaded), then reinstalled — second install skipped the download and completed immediately

Notes

• The fix is intentionally minimal and mirrors the existing pattern in stageLocalBinary (src/installer/orchestrator.zig:754)
• A forced re-download (e.g. for a corrupted binary) can still be triggered by deleting the file from ~/.nullhub/bin/ manually; a --force flag could be added later if needed