[Feature Request] Allow subagents to override reasoning effort in CustomAgentConfig (parity with Claude Agent SDK)

## Summary

Allow custom subagents defined via `CustomAgentConfig` to specify their own reasoning effort independent of the orchestrator session's reasoning. The Claude Agent SDK supports this via an `effort` field on `AgentDefinition`. copilot-sdk's `CustomAgentConfig` does not, so reasoning is fixed at session level and applies uniformly to every subagent invocation.

## Motivation

- Orchestrators often benefit from `high` or `max` reasoning for planning and delegation
- Many subagent tasks are mechanical execution (validation, extraction, format conversion) and do not benefit from deep reasoning, but today pay its latency and cost
- Conversely, security focused or analysis focused subagents may want stronger reasoning than the orchestrator
- Without per subagent control, the tallest reasoning setting propagates to every dispatch

## Current state

- `CustomAgentConfig` (copilot-sdk `types.d.ts`, lines 802 to 833) has no `reasoningEffort` or `effort` field
- Session level `reasoningEffort` in `providerOptions` applies to all subagent dispatches in that session

## Proposed

1. Add optional `reasoningEffort` (or `effort`) field to `CustomAgentConfig`
2. Accept existing values: `low | medium | high | xhigh | max`, plus numeric for models that support it
3. Fall back to session reasoning when omitted
4. Mirror the field in the YAML frontmatter loader (coordinated with github/copilot-cli#2904)

## Precedent

Claude Agent SDK's `AgentDefinition.effort`:
https://code.claude.com/docs/en/agent-sdk/subagents

Identical value vocabulary already in use by this SDK.

## Related

- github/copilot-cli#2904 — Custom Agent YAML Frontmatter Should Support Reasoning Effort (CLI surface for the same feature)
- #1019 — Per agent tool visibility for custom sub agents (same architectural axis)