Fix Ubuntu test prerequisites

[Joe-Nor]

What changed

• Install uv in the Ubuntu test job, matching the phase-diagram CLI test's declared runtime.
• Validate the Julia shim destination before installing DaemonMode, so a request to overwrite the daemon runner itself is rejected before any Julia lookup or installation side effect.

Why

The current main branch and newly synchronized pull requests fail only on Ubuntu for two unrelated infrastructure reasons:

1. scripts/tests/test_phase_diagram.py invokes uv, but the Ubuntu workflow did not install it.
2. scripts/tests/test_julia_daemon.py expects the self-overwrite safety error to take precedence, while the install-shim dispatcher attempted to install DaemonMode first and therefore reported a missing Julia executable.

This infrastructure fix is intentionally separate from challenge submission #196.

Validation

• Targeted regressions: 2 passed.
• Full suite: 235 passed, 9 skipped; 95% reported coverage.
• bash -n scripts/julia-daemon.sh
• git diff --check

[hz-xiaxz]

Thanks for fixing CI for julia-daemon!