Repo instruction files symlinked to targets outside the repo are not loaded #5425

Bug: Repo-level instruction files are not loaded when they are symlinks to targets outside the repository. Regular files with the same content are loaded.

Affected: These files and directories, when symlinked to outside targets: AGENTS.md, CLAUDE.md, GEMINI.md, .claude/CLAUDE.md, .github/copilot-instructions.md, .github/instructions/, and .claude/rules/.

Not affected: Regular files, symlinks to targets inside the repo, and ~/.copilot/copilot-instructions.md when symlinked to an outside target.

Repro:

mkdir -p ~/symlink-instructions-repro/targets ~/symlink-instructions-repro/repo
echo "test instructions" > ~/symlink-instructions-repro/targets/AGENTS.md
cd ~/symlink-instructions-repro/repo && git init

# Symlink to a file outside the repo
ln -s ../targets/AGENTS.md AGENTS.md
copilot instruction list      # no "Repository instructions" section

# Same content as a regular file
rm AGENTS.md && cp ../targets/AGENTS.md AGENTS.md
copilot instruction list      # lists AGENTS.md under "Repository instructions"

Expected: Symlinked instruction files are loaded the same way as regular files.

Version: CLI 1.0.109, Linux.

Related: #4822 (symlink resolution in AGENTS.md discovery; opposite direction), #3264 (symlink docs under ~/.copilot).