fix: preserve active agent across reasoning changes

[mauriciozaffari]

Summary

• include the calling agent in synthetic variant-change prompts
• retain that agent alongside temporary reset state for the idle reset
• add regression coverage for both the set and reset paths

Problem

sendVariantPrompt omitted body.agent. OpenCode therefore assigned synthetic prompts to the configured default agent, which silently switched sessions using a non-default agent after set_reasoning_effort ran.

Verification

• pnpm run format:check
• pnpm run lint
• pnpm run typecheck
• pnpm run test (16 tests)
• pnpm run build
• reproduced against OpenCode 1.18.4 with a custom primary agent and confirmed both the variant-change prompt and subsequent assistant response retained that agent

[mauriciozaffari]

Hi @ian-pascoe, thanks for this plugin, I think it's really helpful.

Would you be able to review this please?

[ian-pascoe]

@mauriciozaffari can you please add a detailed changeset? (run pnpm changeset) make it a patch. Thank you for the contribution!