fix(commands): reject symlink follow during skill install

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Summary

• skilllite add, skill update-from-source, and OpenClaw import copied skill trees with std::fs::copy / Path::is_dir, both of which follow symlinks.
• A skill containing passwd-link -> /etc/passwd (or a directory link to $HOME / ~/.ssh) materialized host file contents into .skills/<name>/. That is an install-time arbitrary file read and can leak secrets if the installed tree is committed, shared, or executed.
• Fix: fail closed on any symlink that would be copied; inspect the source tree before deleting dest so a rejected overwrite cannot wipe an existing skill. Excluded dirs (.venv, node_modules, …) are still skipped even if those names are themselves symlinks.

Task Linkage

• Task ID: TASK-2026-071
• Task folder: tasks/TASK-2026-071-reject-skill-install-symlinks/

Injected Specs

• spec/verification-integrity.md
• spec/task-artifact-language.md
• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/rust-conventions.md
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • python3 scripts/validate_tasks.py
  • cargo fmt --check
  • cargo clippy --all-targets -- -D warnings
  • cargo test -p skilllite-commands --lib -- copy_skill (5 passed)
  • cargo test -p skilllite --test e2e_minimal (3 passed, including e2e_add_rejects_symlink_to_host_file)
  • cargo test -p skilllite-commands / cargo test -p skilllite / cargo test (0 failed)
• Key results:
  • Independent repro: copying ln -s /etc/passwd produced a regular file starting with root:x:0:0
  • After the fix, install errors with symlink is not allowed and dest is left untouched

Regression Scope

• Areas likely affected:
  • skilllite add, update_skill_from_source, import-openclaw-skills
• Explicit non-goals:
  • Agent workspace symlink containment (open PR fix(security): reject symlink and path-containment escapes #132)
  • ZIP symlink entries (already written as regular files containing the path string, not followed)

Docs Sync (EN/ZH)

• Updated EN + ZH docs
• Files:
  • README.md
  • docs/zh/README.md

Review Checklist

• Acceptance criteria in tasks/TASK-2026-071-reject-skill-install-symlinks/TASK.md satisfied (or explicitly deferred)
• tasks/TASK-2026-071-reject-skill-install-symlinks/STATUS.md updated with latest progress
• tasks/TASK-2026-071-reject-skill-install-symlinks/REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date