Standardize MatFormer tier metadata in exported checkpoints

[plugyawn]

Problem

Tiered checkpoint metadata is inconsistent: exported tiered checkpoints can lack matformer_base_intermediate_size and may set matformer_tier=0, which makes auto-detection unreliable for remote repos.

Refs:

• scripts/export_matformer_tiers.py (tier export)

Expected

Tiered checkpoints should always include:

• intermediate_size (sliced)
• matformer_tier (actual tier)
• matformer_base_intermediate_size (base width before slicing)

Possible Approach

• Update export/packaging tooling to write these fields consistently.
• Add validation or a small test to ensure metadata correctness.

Acceptance Criteria

• Exported tiered checkpoints always include correct tier metadata.
• Auto-detection works for both local and remote checkpoints.