Seed model offerings in declared priority order

[pratikbuilds]

Summary

• assign each seeded model offering a distinct provider-major, model-major priority
• make the first curated model for a provider win both Settings and runtime resolution
• cover the fresh Anthropic seed and cross-provider offsets with regression tests

Scope

This implements the fresh-seed priority direction described in #541. It does not add an environment variable for selecting Sonnet, Opus, or another model; ANTHROPIC_API_KEY remains credential-only.

Verification

• bun run check
• 44 hub-client seed tests
• 33 inference resolver tests
• Greybeard and Critique reviews clean

Fixes #541