fix(fetch): use tuple element for MODEL env var in done hint

TIER_REPOS values are tuples ("ENV_VAR", "repo_id"), but _download
called .replace('_REPO','_MODEL') on the whole tuple, raising
AttributeError: 'tuple' object has no attribute 'replace' AFTER the
download had already succeeded. This made every fetch exit 1 even though
the checkpoint was fully written.

Index into the first tuple element (the env var name) before replacing:
TIER_REPOS[tier][0].replace('_REPO','_MODEL').

Verified: both tiers now print the correct EDGE0_*_MODEL export hint and
exit cleanly.