Add natural language / tool-based effort level control (like plan mode)

Summary

Plan mode can be toggled via natural language ("think in plan mode") because Claude has EnterPlanMode/ExitPlanMode tools. However, effort level can only be changed via /effort command or UI — there's no equivalent tool (e.g. SetEffortLevel) that Claude can invoke based on natural language instructions.

Use case

A common workflow is to pair plan mode with high effort and normal mode with medium effort. Currently this requires manual /effort switching each time, which breaks the flow — especially for voice input users who can trigger plan mode naturally but must switch to slash commands for effort.

Proposal

Add a SetEffortLevel tool (or equivalent mechanism) that Claude can call in response to natural language, similar to how EnterPlanMode/ExitPlanMode work. This would enable:

• "Think in plan mode with high effort" → EnterPlanMode + SetEffortLevel(high)
• "Go back to normal" → ExitPlanMode + SetEffortLevel(medium)

Related: #34171