Add explicit worker reasoning variants

[luckeyfaraday]

Summary

• add a persisted worker_variant setting to CLI, MCP workflow, and one-shot delegation paths
• forward OpenCode variants as --variant and log the effective value explicitly
• map Codex variants to model_reasoning_effort, validating none/minimal/low/medium/high/xhigh/max
• reject variants on unsupported backends instead of silently falling back
• document Sol xHigh, Luna Max, and OpenCode/Qwen usage

Verification

• uv run pytest -q (71 passed)
• git diff --check
• bash -n src/master_workflow/scripts/run-opencode.sh
• MCP module import with the optional dependency enabled