Document that --model/-m pins and disables plugin fallback

Summary

--model / -m pins the session model. Plugin-configured models[] / runtime fallback never runs for that invocation. Reasonable, but undocumented as disables failover, so users who set a backup gateway never see it when they also pass -m.

Repro

1. Plugin fallback chain: gateway-a/model then gateway-b/model.
2. opencode run -m gateway-a/model --agent sisyphus ping while gateway-a returns 429.

Actual

No hop to gateway-b.

Expected

Document that -m pins and skips fallback. Optionally warn once when -m is combined with a plugin that advertises fallback models.

Suggested fix

Docs plus a single warning. Do not change pin semantics unless maintainers want an opt-in --allow-fallback.