fix: support current V2 command API

[danyel117]

Summary

• migrate V2 slash-command registration from removed draft.get/draft.update methods to draft.add
• execute /goal by forwarding the expanded command prompt through context.session.prompt while preserving attachments and delivery
• validate against @opencode-ai/plugin@0.0.0-beta-18593 and align OpenTUI dependencies with the beta SDK
• add a regression test that invokes the registered command

Verification

• bun run test (213 passing)
• bun run lint
• bun run typecheck
• bun run build
• bun run pack:dry-run