[FEATURE] Allow changing effort on a running subagent via SendMessage

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Effort can be set when a subagent is launched (follow-up to #43083), but not changed afterwards. SendMessage has no effort parameter, so raising the effort of a long-running agent means stopping and relaunching it, which discards its context. The main conversation already supports changing effort between turns with /effort, so subagents behave inconsistently.

Proposed Solution

Add an optional effort parameter to SendMessage, applied from the subagent's next turn. Show each subagent's current effort level in the agent list, not only inside its transcript.

Alternative Solutions

Relaunching at higher effort and re-supplying context manually: slow, lossy, and costs extra tokens.

Priority

Medium - Moderate impact on productivity

Feature Category

API and model interactions

Use Case Example

A subagent starts on a task that looks simple, then hits a harder sub-problem. Today the only options are to accept lower-effort reasoning or to relaunch it and lose its accumulated context.

Additional Context

_No response_