Nested agents load ALL copilot-instructions.md, not just the ones at the nested level

### Describe the bug

This feature was added: https://github.com/github/copilot-cli/issues/1655

However, whilst nested agents are now dispalyed, they read the `copilot-instructions.md` at ALL levels.

This is not expected behavior because it should be assumed that nested folders are sub-projects / workspaces in their own right

Solutions:
- Default to only read the copilot-instructions.md at the same level in a nested hierarchy
- Add a boolean setting to turn this behavior on / off as needed, defaulted to off

### Affected version

_No response_

### Steps to reproduce the behavior

create:
- .github/copilot-instructions.md
- nested_folder/.github/agents/test-agent.md
- nested_folder/.github/copilot-instructions.md

ask the test-agent.md to link the absolute path to all of the instructions in its context window & memory and it will link both:

### Expected behavior

it should be assumed that nested folders are sub-projects / workspaces in their own right. therefore it shouldn't be loading in content that's outside the view and/or control of the hierarchy

### Additional context

_No response_

Start by reproducing the issue with .github/copilot-instructions.md, nested_folder/.github/agents/test-agent.md, and nested_folder/.github/copilot-instructions.md. Trace the nested-agent instruction discovery entry point and any existing tests for instruction loading; done means the nested agent uses only the instruction file at its own level by default, without loading the parent file.

索引モデルが issue の本文から書いたものです。