MPS: forward --model to Claude Code for a relayed provider

[masonc08]

Problem

ucode claude --provider <mps> --model <m> is ignored for a relayed (Claude subscription) Model Provider Service. A customer on a Claude Enterprise relay needs to launch on a specific model (e.g. claude-opus-4-8[1m]) without Claude Code's /model picker (limited list) or editing settings.json (blocked under MDM).

What we learned

• In the AI Gateway relayed Anthropic path (ai-gateway/src/v3/client/ExternalModelDirectClient.scala), the client's model is passed through to Anthropic; Team/Enterprise validates it against the allowlist, personal Max skips that — the gateway never selects a model server-side.
• Real Enterprise relays are allow_all_targets with no declared Claude targets. So there's nothing for ucode to resolve --model against — an approach that pins the gateway env from declared targets can't help them.
• The proven lever is Claude Code's own --model: ucode claude --provider <mps> -- --model opus already launches on Opus via the subscription (verified e2e against a relayed Enterprise MPS on staging).

Change

For a relayed provider, forward --model to Claude Code's own --model launch arg — exactly what the -- --model X passthrough does, now automatic. Claude Code selects the model natively and the relay honors it; no declared targets required, and it sidesteps the ANTHROPIC_MODEL client-side check (we use Claude Code's native flag). A --model that already rode in after -- wins. Non-relayed providers (API-key / Bedrock) keep the env-pinning path from #380 unchanged.

The change is cli.py-only: drop the relayed "--model is ignored" warning, guard the env-pinning resolve with not relayed, and inject --model into the launch args for a relayed provider (surfaced in the launch summary via the existing forwarded_model).

Testing

• Unit: test_cli.py relayed cases (forwarded-to-Claude-Code, and no---model forwards nothing); the provider suites, test_agents_init, test_databricks, test_agent_claude green (729). ruff clean.
• e2e: ucode claude --provider <relayed_enterprise_mps> --model opus launches on Opus (matches the proven -- --model opus passthrough).

This pull request and its description were written by Isaac.