[Feature Request] Add subagent-scoped effort control (CLAUDE_CODE_SUBAGENT_EFFORT env var and settings key)

Bug Description
Feature request: subagent-scoped effort control (parity with subagent model override)

There's an asymmetry between model and effort configuration for subagents:

• Model can be forced for all subagents independently of the main agent via CLAUDE_CODE_SUBAGENT_MODEL.
• Effort has no subagent-scoped equivalent. There's no CLAUDE_CODE_SUBAGENT_EFFORT env var, no subagent effort key in settings.json, and no --subagent-effort flag. CLAUDE_CODE_EFFORT_LEVEL / --effort / effortLevel are all session-wide and also govern the main agent, and the env var outranks /effort so it disables interactive control.

The only per-subagent effort control is the effort: frontmatter field, which is per-definition and doesn't reach the file-less built-in subagents (general-purpose, Explore, Plan, etc.), since those inherit the session effort.

Ask: a subagent-scoped effort lever mirroring CLAUDE_CODE_SUBAGENT_MODEL — e.g. a CLAUDE_CODE_SUBAGENT_EFFORT env var (and/or a subagentEffort settings key). Use case: run subagents at a different effort level (e.g. xhigh on Opus) while keeping the main agent's effort freely adjustable via /effort (e.g. high on Fable).

Environment Info

• Platform: linux
• Terminal: kitty
• Version: 2.1.215
• Feedback ID: 4937e72f-9822-4b0a-9e85-737a9b42ac94

Errors
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