Set Antigravity `save_dir` to substrate's persistent mount

[joycel-github]

Antigravity SDK defaults save_dir to tempfile.mkdtemp(prefix="antigravity_") → /tmp/antigravity_xxx/{sdk_conv_id}.db. This works as substrate snapshot includes /tmp.

Better design: once substrate team lands the persistent home dir mount, set save_dir to that path rather than rely on implicit /tmp snapshotting.

Not milestone 1.

[rakyll]

@joycel-github This is fixed, right?