fix(sandcastle): wire reasoning effort through to agent providers

fix(sandcastle): wire reasoning effort through to agent providers

Pass AGENT_*_EFFORT constants through agentProvider() to the opencode
provider's variant flag. LoopStrategy gains actorEffort/criticEffort
optional overrides following the same pattern as actorModel/criticModel.

- opencode: effort mapped to --variant CLI flag
- pi: no effort support (provider limitation, param silently ignored)