fix(agent): track visited realpaths in iter_skill_index_files to prevent symlink loops

[CarlitoDon]

Summary

When building the system prompt and scanning skill directories, iter_skill_index_files uses os.walk(..., followlinks=True). If a skill directory contains a circular or self-referential symlink (e.g. dir/symlink -> dir), os.walk recursively traverses the directory indefinitely, producing hundreds of duplicated skill matches and inflating system prompts with repeated skill descriptions.

Fix

• Added visited_dirs: set[str] in iter_skill_index_files that tracks os.path.realpath(root).
• If an ancestor or symlink destination realpath has already been visited during traversal, subdirectories are pruned (dirs[:] = []) to prevent infinite recursion.
• Added comprehensive unit test in tests/agent/test_skill_utils.py to verify symlink loop protection.

[Copilot]

Pull request overview

This PR hardens skill-directory scanning during system-prompt construction by preventing os.walk(..., followlinks=True) from recursing indefinitely when encountering circular/self-referential symlinks, which previously could duplicate skill matches and bloat prompts.

Changes:

• Track visited directory realpaths during iter_skill_index_files() traversal and prune recursion when a realpath repeats.
• Add a unit test that creates a self-referential symlink and asserts traversal remains finite and non-duplicative.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/skill_utils.py	Adds realpath-based visited directory tracking to prevent symlink-induced recursion loops while walking skill directories.
tests/agent/test_skill_utils.py	Adds a regression test covering circular symlink handling during skill index file iteration.

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