MPS: launch on a servable default model, and honor --model under --provider

[masonc08]

Problem

ucode claude --provider <mps> always started on Claude Code's built-in "family default" (opus). When a Model Provider Service declares no opus target, the gateway 403s that default — so a bare launch failed for any service that only offers sonnet/haiku (e.g. an API-key Anthropic MPS scoped to sonnet+haiku). --model couldn't rescue it either: --model + --provider was rejected outright with "Use either --model or --provider, not both."

Stacked on #376 (which pins the declared per-family targets); this makes the launch model land on one of them.

Change

Under a provider:

• Auto-pick the best servable tier. With no --model, choose the most capable tier the service actually declares (opus > sonnet > haiku) and pin it via ANTHROPIC_MODEL, so the session starts on a servable model instead of dead-ending on opus. When the service does offer opus, pin nothing — Claude Code's own default already works, and this avoids the duplicate /model picker row ANTHROPIC_MODEL produces.
• Honor --model (guard removed). A family alias (opus/sonnet/haiku) resolves to that tier's declared target (clear error if the service doesn't offer it); any other value is trusted as a raw target id. Routed via route_root_model, not custom_model, so it doesn't clobber the per-family pins from Pin declared Anthropic MPS targets by family, like Bedrock #376.
• Relayed services: model selection is server-side, so --model is ignored with a warning.

The launch banner now also prints the chosen Model: under a provider.

Known gap (deferred)

For an opus-less Bedrock service the region-prefixed slug can be rejected by ANTHROPIC_MODEL's client-side check. That case was already broken (bare launch 403s on opus), and opus-having Bedrock services are unaffected (they return None and behave as today). The follow-up fix is to pin the servable target into the opus family slot instead — that channel is passed through unchecked.

Testing

• Unit: resolve_provider_launch_model (7 cases: opus-present no-op, sonnet/haiku fallback, alias→target, unavailable-tier error, raw-id passthrough); CLI provider-launch (alias pin, auto-default, opus-present no-op, unavailable-tier error, relayed-ignored).
• Suites green: test_databricks.py (248), test_cli.py (218), test_agent_claude.py + test_agents_init.py (188). ruff clean.
• End-to-end against an API-key Anthropic MPS (canonical target names): recommended sanity check — bare launch starts on sonnet with no 403; --model haiku starts on haiku; --model opus errors clearly.

This pull request and its description were written by Isaac.