fix(cli): honor agent model across run targets

[opencode-agent]

Summary

• add an opt-in session.select operation with explicit preserve, configured, and explicit model policies
• resolve configured agent models under SessionLocationMiddleware after a bounded PluginSupervisor.flush
• atomically publish agent/model Session changes and skip redundant durable selection events
• apply explicit run selections after fresh, continued, explicit, or forked target resolution
• preserve generic session.create({ agent }) model independence and defer missing-agent rejection to execution

Tests

• bun test test/session-create.test.ts in packages/core (35 passed)
• bun test test/session.test.ts test/model.test.ts in packages/server (2 passed)
• bun test test/run/run.test.ts test/session-target.test.ts test/mini.test.ts test/run/noninteractive.test.ts in packages/cli (31 passed)
• bun test test/promise.test.ts test/effect.test.ts in packages/client (29 passed)
• bun typecheck in packages/core, packages/cli, packages/protocol, and packages/client
• scoped lint (0 errors) and Prettier checks
• regenerated client surfaces with bun run generate

Fixes #42561

Requested by: @neriousy (Filip via Slack)