Fixes #18675.

Summary

• replace the unbounded fallback rglob("*") supporting-file scan with a deterministic scanner that prunes dependency/cache directories before descent
• cap supporting-file hints at 200 entries and emit a truncation note outside the file-path list
• skip empty files in the fallback scan
• add regression coverage for nested node_modules, empty files, and the 200-file cap

Verification

• scripts/run_tests.sh tests/agent/test_skill_commands.py::TestSkillDirectoryHeader::test_fallback_supporting_file_scan_skips_dependency_dirs_and_caps
• scripts/run_tests.sh tests/agent/test_skill_commands.py
• git diff --check origin/main...HEAD