feat(core): expose session model switching

[kitlangton]

Summary

• expose the existing durable Core sessions.switchModel operation through the public OpenCode facade
• preserve the typed missing-session error and exact model reference
• enable embedded consumers such as OpenCord to switch a session model without reaching into internal services

Checks

• OPENCODE_DB=:memory: bun run test public-opencode.test.ts
• bun run typecheck in packages/core
• repository typecheck commit hook
• bunx prettier --check packages/core/src/public/opencode.ts packages/core/src/public/session.ts packages/core/test/public-opencode.test.ts
• git diff --check origin/dev...HEAD