Add Grok 4.6 to Smith and router catalogs

[edwin-zvs]

Summary\n\n- add grok-4.6 to the shared Smith/router model catalog for API-key and Grok subscription routes\n- make Grok 4.6 the default in built-in routing and Smith configure flow\n- use the documented 500k context window in Smith\n- refresh Grok examples and current-model test fixtures\n\n## Verification\n\n- xAI API docs list model slug grok-4.6 with a 500k context window\n- installed grok models reports grok-4.6 as the subscription default\n- cargo test -p construct-protocol model_completions_are_offered_after_model_command\n- cargo test -p construct-adapter-smith grok_4_6_gets_its_advertised_context_window_on_both_auth_paths\n- cargo test -p construct-daemon seed_models_come_from_the_shared_catalog\n- cargo test -p construct-daemon every_direct_api_key_provider_is_a_builtin_target\n- cargo test -p construct-daemon auto_unavailable_when_only_a_subscription_credential_exists\n- cargo test -p construct-daemon grok_and_kimi_advertise_their_native_scales\n- cargo test -p construct-daemon effort_support_matches_subscription_model_capabilities\n- cargo build\n\n## Formatting\n\ncargo fmt --all -- --check is already red on unrelated files in current main; git diff --check passes for this branch.\n\nNo visual artifact: this is a model-catalog/default refresh with no layout or rendering change.