[codex] fix(skills): load symlinked markdown skills

[yzlu0917]

Summary

• load project and global markdown skills from symlinked skill directories in both the CLI and core loaders
• keep read_skill working by enumerating first-level skill directories instead of relying on recursive directory walks that skip symlinks
• add regression tests covering symlinked .continue/skills/<skill> directories for both loaders

Why

npx skills add ... --skill <name> can install skills into a project via symlink. Before this change, those symlinked skill directories were skipped during skill discovery, so CLI skill listings and the read_skill tool could not find the installed skill.

Validation

• ran npm test -- src/util/loadMarkdownSkills.test.ts in extensions/cli
• ran npm run vitest -- config/markdown/loadMarkdownSkills.vitest.ts in core
• ran npm run lint in extensions/cli (passes with 2 pre-existing warnings)
• ran git diff --check

Closes #11985

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Summary by cubic

Include symlinked markdown skills in discovery for both core and the CLI so skills installed via symlink show up and are readable by read_skill. Adds tests to prevent regressions.

• Bug Fixes
  • Discover SKILL.md in symlinked directories under project and global skills dirs.
  • Switch to first-level directory enumeration (includes symlinks) instead of recursive walks that skipped them.
  • Add core and CLI tests for symlinked .continue/skills/<skill> and verify read_skill returns content and files.

^{Written for commit 3a93696. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files