Regression: COPILOT_CUSTOM_INSTRUCTIONS_DIR not loading all instructions

### Describe the bug

In v1.0.9, instruction files specified via COPILOT_CUSTOM_INSTRUCTIONS_DIRS are not being loaded. This
worked correctly in v1.0.8.

Environment:

- COPILOT_CUSTOM_INSTRUCTIONS_DIRS points to 7 directories, each containing
.github/instructions/team.instructions.md with applyTo frontmatter
- Working directory is a multi-root workspace that is not a git repository

### Affected version

v1.0.9

### Steps to reproduce the behavior

1. Set COPILOT_CUSTOM_INSTRUCTIONS_DIRS to one or more directories containing
.github/instructions/*.instructions.md
2. Launch the CLI in a directory that is not a git repo root
3. Run /instructions

### Expected behavior

Expected: All instruction files from COPILOT_CUSTOM_INSTRUCTIONS_DIRS appear in the list.

### Additional context

_No response_