fix(security): reject symlinks when fetching skills from a git repo

[albertodebortoli]

Description

• What & why: GitRepositorySkillFetcher fetches skills by shallow-cloning a repository and reading files from the clone. It previously skipped only symlinks that resolved to directories, so a file symlink in a malicious repo (e.g. leak -> /Users/you/.ssh/id_rsa) would be enumerated and read, copying local file content outside the repository into the skill cache. downloadSkill also resolved symlinks before reading without verifying the result stayed inside the clone.
• Fix: The fetcher now skips all symbolic links when enumerating skill paths, and downloadSkill refuses to read a file that is a symlink or whose resolved path escapes the clone directory. This matches GitHubSkillTreeClient, which already filters symlinks, keeping the two fetchers consistent.
• Trade-offs: Skills that legitimately rely on symlinked files will have those entries skipped; this mirrors the existing GitHub-API fetcher's behaviour, so it is consistent across transports.

Type of Change

• Bug fix
• Feature
• Maintenance / Refactor
• Documentation
• CI / Tooling
• Other (specify)

How Has This Been Tested?

swift build and swift test locally on macOS (arm64).

• Added / updated unit tests
• Manually tested locally
• Tested on macOS (arch: arm64)

New tests: a file symlink in a cloned repo is excluded from enumerated skill paths, and downloadSkill throws fileReadFailed when asked to read a symlinked path pointing outside the clone. The existing directory-symlink test still passes.

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• Documentation updated (comments on the symlink handling)

Breaking Changes?

• No

Additional Notes

Part of a set of independent supply-chain hardening changes; this PR stands alone and can be merged on its own.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...sitorySkillFetcher/GitRepositorySkillFetcher.swift	93.75%	1 Missing ⚠️

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