Remove Nova Premier (us.amazon.nova-premier-v1:0) from Studio model picker before its Sept 14, 2026 EOL

## Summary

`us.amazon.nova-premier-v1:0` (Amazon Nova Premier) is scheduled for end of life on Amazon Bedrock on **September 14, 2026**. It is still offered as a selectable model in the Studio model picker. After the EOL date any agent built with it will fail at first invoke with a model-not-available error, so the entry should be removed before then.

`anthropic.claude-opus-4-1-20250805-v1:0` (Claude Opus 4.1) is also on the EOL list (January 8, 2027) but is not referenced anywhere in this repo, so no action is needed for it.

## Where Nova Premier is referenced (main)

Model catalog entries (user-facing, should be removed):

- `frontend/src/studio/lib/models.ts:83` — `{ model_id: 'us.amazon.nova-premier-v1:0', model_name: 'Amazon Nova Premier v1' }`
- `apps/studio/src/components/property-panel.tsx:129` — same entry in the legacy Studio app

Model-specific handling (can be removed once the catalog entry is gone, or left as harmless dead branches):

- `frontend/src/studio/PropertyPanel.tsx:609` — `if (modelId.includes('nova-premier'))`
- `frontend/src/studio/lib/code-generator.ts:1498` and `:1604` — `max_tokens` clamp to 32000
- `frontend/src/studio/lib/graph-code-generator.ts:94` — same clamp
- `docs/studio-integration.md:295` — documents the clamp

`DEFAULT_MODEL_ID` is unaffected (it is Sonnet 5), and no backend, evaluation, A/B, or governance code path references Nova Premier. Impact is limited to users who manually pick it from the dropdown.

## Suggested change

1. Remove the two catalog entries so the model can no longer be selected.
2. Drop the `nova-premier` special-casing in `PropertyPanel.tsx`, `code-generator.ts`, `graph-code-generator.ts`, and the note in `docs/studio-integration.md`.
3. Optionally, show a warning in the property panel when loading a saved flow whose node still carries `us.amazon.nova-premier-v1:0`, so existing flows are not silently broken.

## Reference

Amazon Bedrock model lifecycle: https://docs.aws.amazon.com/bedrock/latest/userguide/model-lifecycle.html

Start with the model catalog entries in frontend/src/studio/lib/models.ts and apps/studio/src/components/property-panel.tsx, then inspect the Nova Premier branches in frontend/src/studio/PropertyPanel.tsx, the code generators, and docs/studio-integration.md. Remove the obsolete catalog and model-specific references as scoped, and verify Nova Premier is no longer selectable while unrelated model handling remains intact.

Written by the indexing model from the issue text.