feat(skills): configurable SSOT storage location (cc-switch | unified), align with upstream GUI

[ShatterDusk]

Summary

Add configurable skills SSOT storage location (skill_storage_location: cc-switch | unified) to the CLI, aligning with the upstream GUI's skillStorageLocation feature (farion1231/cc-switch). In unified mode, ~/.agents/skills/ becomes the SSOT — the cross-tool standard directory adopted by skills.sh, LangChain, and six major harnesses.

Closes #413

Motivation

~/.agents/skills/ is the de-facto cross-harness standard for agent skills (Codex, Gemini CLI, DSH, OpenCode, Windsurf, Warp scan it natively; LangChain deepagents and skills.sh adopt it). The CLI's SSOT was hardcoded to {config_dir}/skills, forcing users of the .agents convention into two divergent skill copies. See #413 for the full harness adoption matrix.

Changes

File	Change
src/services/skill.rs	SkillStorageLocation enum; get_ssot_dir() honors location; migrate_storage() with pre-migration backup; validate_skill_storage_destination()
src/settings.rs	skill_storage_location field (#[serde(default)]), get/set functions
src/lib.rs	re-exports for tests/callers
src/cli/commands/skills.rs	skills storage-location command (get/set + triggers migration)
tests/skills_service.rs	tests for default, roundtrip, storage-location CLI

CLI usage

cc-switch skills storage-location              # show current (cc-switch | unified)
cc-switch skills storage-location unified      # switch to ~/.agents/skills + migrate
cc-switch skills storage-location cc-switch    # switch back + migrate

Behavior (aligned with upstream GUI's migrate_storage)

1. Pre-migration backup of the old SSOT to {config_dir}/skill-backups/ (new: upstream has no backup; added for rollback safety)
2. Validate destination (rejects aliased app skills dirs via canonicalize)
3. Move skills one-by-one (rename → copy+delete fallback; soft-fail per skill)
4. Update setting only after files moved
5. Re-sync app skill dirs to the new SSOT

Testing

• New: 5 unit tests (migrate_storage_*, get_ssot_dir_switches_on_location) + 3 integration tests (storage_location_*) — all pass
• services::skill module: 20/20 pass
• Full lib: 4088 passed; 16 failed — all 16 failures are pre-existing cli::tui::theme::tests (terminal color detection fails in non-TTY env), unrelated to this change (verified on clean baseline via git stash)
• cargo fmt --check clean; cargo clippy no new warnings

Notes

• Values: clap kebab-case cc-switch/unified for CLI args; serde snake_case cc_switch/unified in settings.json
• Migration backup uses directory copy (not zip) for simplicity — sufficient for rollback
• migrate_storage traverses the SSOT directory (not the index), so traversal-safe by construction