Let the models directory be set independently of NODESPACE_HOME #3791

Overview

Let the models directory be set on its own, so a redirected NODESPACE_HOME can keep using the real downloaded models.

Problem Statement

models_dir() in packages/daemon/src/lib.rs is always NODESPACE_HOME/.nodespace/models. That is right for isolating a database and a registry, but a run that isolates the home (an eval, a test harness) then finds no models, and the daemon downloads a multi-GB second copy of each into the throwaway home. The workaround is a symlink from the isolated home to ~/.nodespace/models, which has to be remembered and is easy to get wrong.

Proposed Solution

A NODESPACE_MODELS_DIR variable. When it is set and not empty, models_dir() returns it; otherwise it resolves under the NodeSpace home as today. Every consumer already goes through models_dir(), so nothing else changes. The eval runbook then sets the variable instead of linking a directory.

Acceptance Criteria

• NODESPACE_MODELS_DIR set and non-empty is the models directory, whatever NODESPACE_HOME is
• Unset or empty resolves under the NodeSpace home exactly as before
• No consumer computes the models path any other way
• The eval runbook sets the variable instead of linking the directory

Related Issues

• Eval preflight cannot confirm guidance provenance or that no other model is on the GPU #3788 (eval preflight, which this makes cheaper to run)