feat(cursor-proxy): support model selection via CURSOR_MODEL env var

[aulorbe]

Summary

Stacked on #3420.

The Cursor proxy previously hardcoded "openrouter/auto" for all OpenRouter API calls, always reported Claude Sonnet 4.6 as the default model, and offered no way to pick a different model.

This PR:

• Model env var: adds modelEnvVar: "CURSOR_MODEL" to the Cursor agent config so the orchestrator injects the user's model choice into .spawnrc
• Dynamic model list: replaces the hardcoded model list in the proxy with a dynamic fetch from OpenRouter's /api/v1/models endpoint (cached after first fetch, falls back to a default if it fails)
• Proxy model routing: the bidi proxy reads CURSOR_MODEL from the environment and forwards it to OpenRouter; the unary proxy's GetDefaultModelForCli and GetUsableModels responses reflect the configured model
• Searchable model picker: replaces the freeform "Model ID" text input in the interactive setup flow with a searchable autocomplete picker populated from OpenRouter's model list (falls back to text input if the fetch fails)

Users can set their preferred model via:

1. The "Custom model" toggle in the interactive setup options (now shows a searchable picker)
2. MODEL_ID env var
3. ~/.config/spawn/preferences.json

Test plan

• bunx @biomejs/biome check src/ passes
• bun test src/__tests__/cursor-proxy.test.ts — all 13 tests pass
• bun test src/__tests__/cmd-interactive-cov.test.ts — all 10 tests pass
• Manual: toggle "Custom model" in setup options → verify searchable picker appears
• Manual: pick a model → verify it's used in the Cursor CLI session
• Manual: test with fetch failure (offline) → verify fallback text input works

[aulorbe]

• feat(cursor-proxy): support model selection via CURSOR_MODEL env var #3421 👈 (View in Graphite)
• macos-local-cursor-cli-bug-fix

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