Support symlink for skills discovery. (#8801)

Skills discovery now follows symlink entries for SkillScope::User
($CODEX_HOME/skills) and SkillScope::Admin (e.g. /etc/codex/skills).

Added cycle protection: directories are canonicalized and tracked in a
visited set to prevent infinite traversal from circular links.

Added per-root traversal limits to avoid accidentally scanning huge
trees:
- max depth: 6
- max directories: 2000 (logs a warning if truncated)

For now, symlink stat failures and traversal truncation are logged
rather than surfaced as UI “invalid SKILL.md” warnings.

Author: xl-openai