feat(skills): add configurable storage location

[SaladDay]

Reimplements and supersedes #414 while preserving credit for @ShatterDusk's original contribution.

Summary

• add configurable managed Skills storage at ~/.cc-switch/skills or ~/.agents/skills
• align migration behavior with the upstream GUI backend, with conflict checks and recoverable journaling
• add the CLI command and family-style TUI setting/picker/help flow
• keep default storage and cloud behavior unchanged; guard Unified archives from unmanaged content

Validation

• full library suite: 4161 passed, 0 failed, 2 ignored
• skills service integration tests: 26 passed
• WebDAV/S3 settings and archive integration tests: 24 passed
• cargo fmt --check, cargo check --bin cc-switch, cargo clippy --lib --no-default-features, and git diff --check
• isolated CLI round-trip between both storage locations
• multi-round independent blind review followed by a fresh convergence review: no in-scope blockers