feat(skills): add archive and restore subcommands

[amirulabu]

Summary

Adds two new subcommands to wukong skills for temporarily deactivating skills without deleting them:

• wukong skills archive — interactive multi-select of installed skills; moves each .agents/skills/<slug> → .agents/skills-archive/<slug> (per scope: project/global) and removes the .claude/skills/<slug> symlink so agents no longer see it.
• wukong skills restore — interactive multi-select of archived skills; moves them back into .agents/skills/<slug> and recreates the .claude/skills/<slug> symlink via the existing create_skill_symlink().

This is a non-destructive alternative to skills remove. The mv-manifest.json is left untouched so update-tracking is preserved across archive/restore.

Details

• Uses fs::rename (same root/filesystem → atomic, simple).
• Skips already-archived / already-active slugs with a warning.
• Local-only commands (no registry/config client needed), matching the remove pattern.
• Core file operations are extracted into pure archive_skill() / restore_skill() helpers so they can be unit-tested independently of the interactive prompt.

Follow-up hardening

• Idempotent restore. Restore now removes any stale .claude/skills/<slug>/SKILL.md before recreating the symlink. Previously, a leftover link (e.g. from a partially-failed archive) made create_skill_symlink() fail with EEXIST, so the skill was moved back and the archive dir deleted but the command reported Failed — leaving a half-restored state that re-runs never repaired ("already active"). Restore is now safe to re-run.
• No config required for local-only commands. All skills subcommands previously routed through a context builder that hard-failed if ~/.config/wukong/.../config.toml was missing (Run wukong init), even though archive/restore/remove/init touch no network or auth. Added a config-tolerant get_skills_context() so local-only commands run without wukong init; the registry-backed commands (list/find/add/update/publish) still load config themselves and surface the same error if it's missing.
• Self-documenting archive folder. archive writes a README.md into .agents/skills-archive/ explaining what the folder is and that the Wukong CLI created it, so the (intentionally reversible) folder isn't a mystery — including the empty parent left behind after a full restore.

Test plan

• cargo fmt --all ✅
• cargo clippy -- -D warnings ✅
• cargo test --test skills and --test completion ✅ (completion snapshots regenerated for the two new subcommands)
• New unit tests over the extracted helpers ✅ — archive move + link removal + README, archive-conflict skip, restore round-trip + symlink, restore idempotency with a stale .claude link (regression for the fix above), restore already-active skip.
• Manual end-to-end (via pty): archive moves the skill dir + drops the symlink and writes the README; restore moves it back and the symlink resolves again; both run with no config file present; re-running hits the skip paths.

Note: a full cargo test on my machine has unrelated linker failures in other test binaries (ld: library 'clang_rt.osx' not found, broken local Command Line Tools — toolchain looks under clang/17, system has clang/21) — environmental, not from this change. CI should be unaffected.