Regression: COPILOT_CUSTOM_INSTRUCTIONS_DIR not loading all instructions

[brettcurtis]

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