workspace hygiene hides disk pressure when size scan is disabled

[chubes4]

Problem

workspace hygiene reported workspace_size=0 B because size scanning was disabled, while /Users/chubes/Developer had hundreds of GiB of reclaimable artifacts. The direct disk scan found Homeboy Rust target/ directories totaling about 579 GB.

Evidence

• studio wp datamachine-code workspace hygiene showed workspace_size=0 B, size_mode=disabled, and disk_free=235.9 GiB.
• Direct measurement with du -sh /Users/chubes/Developer/* showed many homeboy@.../target hotspots, including 8-12G worktrees.
• After artifact cleanup, free disk rose to 548.8 GiB.

Expected

Hygiene should not look clean while size mode is disabled and the workspace is under heavy disk pressure. It should either show an explicit unknown size state or provide a cheap hotspot probe/suggestion that points operators at large rebuildable artifacts.

Impact

Operators chase worktree count/lifecycle issues while the real disk pressure is artifact directories.

Acceptance criteria

• workspace hygiene does not display workspace_size=0 B as if it is measured when size scan is disabled.
• Hygiene includes a clear warning or cheap bounded hotspot hint when disk pressure exists and size mode is disabled.
• Output points to a safe next command for size/hotspot discovery.