fix(skills): prevent infinite loop on cyclic symlinks in iter_skill_index_files

[liuhao1024]

What does this PR do?

iter_skill_index_files() uses os.walk(followlinks=True) but has no cycle detection. When the skill directory tree contains cyclic symlinks (e.g. A → B → A or self-referencing directories), os.walk enters an infinite loop, causing the agent to hang during skill scanning.

Related Issue

Fixes #18809

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• See commit messages for detailed changes

How to Test

1. Run pytest tests/ -q — all tests should pass
2. Verify the specific scenario described above is resolved

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.4.1

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture and workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A

[alt-glitch]

Likely duplicate of #18809 — same cyclic symlink bug in iter_skill_index_files(). Also related to #7634 which proposes the same cycle guard fix.

[alt-glitch]

Likely duplicate of #18809 — same cyclic symlink bug in iter_skill_index_files(). Also related to #7634 which proposes the same cycle guard fix.

[teknium1]

Thanks for addressing a real regression introduced when symlink following was enabled in 02aba4a728e26b4756d2932bafd890d663f6328a.

Problems

• The guard only covers iter_skill_index_files, but current main also walks skill directories with followlinks=True in agent/prompt_builder.py:1281. build_skills_system_prompt() reaches that unguarded manifest traversal through snapshot validation at agent/prompt_builder.py:1314, and through cold-path snapshot creation at agent/prompt_builder.py:1566. A cyclic skill tree can therefore still block prompt startup.
• The PR tests cover only iter_skill_index_files; add coverage for the prompt snapshot path. The added tests also call symlink_to() without the unavailable-symlink handling used by tests/agent/test_skill_commands.py:47-49 and tests/agent/test_skill_utils.py:376-378.

Suggested changes

• Use one cycle-safe traversal strategy for both agent/skill_utils.py:796 and agent/prompt_builder.py:1281, preserving intended symlink discovery.
• Add cold and snapshot-validation regression coverage through build_skills_system_prompt().
• Skip the symlink tests when the platform/filesystem cannot create them.

Automated hermes-sweeper review.