fix: change Antigravity field from map to array to match remote models.json format

[I3eg1nner]

Problem

The binary built from main panics on startup with:
panic: registry: failed to parse embedded models.json: embed: decode models
catalog: json: cannot unmarshal array into Go struct field
staticModelsJSON.antigravity of type
mapstring*registry.AntigravityModelConfig

Root Cause

The remote models.json (maintained at router-for-me/models) changed the antigravity section from a map format to an array format — consistent with all other sections (claude, gemini, codex-*, etc.). However, staticModelsJSON in internal/registry/model_definitions.go still declared Antigravity as map[string]*AntigravityModelConfig.
This is a runtime contract mismatch — Go compilation succeeds regardless, so CI cannot catch it.

Changes

• Add AntigravityModelEntry struct with an ID field to support array-based model entries
• Change staticModelsJSON.Antigravity from map[string]*AntigravityModelConfig to []*AntigravityModelEntry
• Update GetAntigravityModelConfig() to iterate over the array
• Update LookupStaticModelInfo() to search the array by ID
• Update validateAntigravitySection() to validate array entries, now with duplicate ID detection

Verification

The commit compiles cleanly and passes all existing tests. The remote models.json at https://raw.githubusercontent.com/router-for-me/models/refs/heads/main/models.json now parses successfully.