fix(skills): prevent symlinked install targets

[levineam]

Motivation

• Close a symlink-following and TOCTOU filesystem write vulnerability in the skill installer by ensuring installs cannot be redirected outside the intended destination root via symlinked path components or symlinked targets.

Description

• Add validation to resolve and assert the installation root is a real directory, owned by the current user, and not group/other-writable via prepareInstallDirectory and assertSafeInstallDirectory in modules/jarvos-skills/src/index.js.
• Reject any symbolic-link components inside the requested destinationDir with assertNoSymlinkComponents, and ensure per-skill directories are created and real-checked with prepareSkillDirectory to enforce containment under the resolved root.
• Replace the previous naive fs.copyFileSync flow with copySkillFileSync which opens targets with O_NOFOLLOW and uses O_EXCL (unless force is set) to avoid races and prevent following target symlinks, and update installSkills to preflight and reject non-regular or symlinked targets before writing.
• Use the resolved realpath of the destination for emitted events and add regression tests in modules/jarvos-skills/test/skills.test.js that verify symlinked directories and symlinked target files cannot redirect installation.

Testing

• Ran the package test suite with npm test inside modules/jarvos-skills, and both @jarvos/skills bundle and projection tests passed.
• Confirmed the new regression tests exercise symlinked-directory and symlinked-target scenarios and that installSkills rejects them as expected.
• Performed git diff --check to ensure no whitespace or diff errors were introduced and found none.

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Codex Task