clean-cachelisted

# Clean cache Opt-in destructive command. Deletes the persistent local clone cache that DISCOVER and REFRESH populate under `~/.cache/skill-engine/`. Always dry-runs first; only deletes after explicit user confirmation. ## Usage `/skill-engine:clean-cache [kind]` `kind` is optional: - omitted → clean every cache subtree under `~/.cache/skill-engine/`, including old flat-layout entries. - `git-managed` → clean only `~/.cache/skill-engine/git-managed/`. - `web-doc` → clean only `~/.cache/skill-engine/web-doc/`. Always prompts with a list of directories before deletion. No deletion happens without confirmation. ## What gets deleted Everything under `${XDG_CACHE_HOME:-$HOME/.cache}/skill-engine/`. The cache is partitioned by source kind: - `~/.cache/skill-engine/git-managed/<source_id>-<sha>/` — shallow clones of git-backed upstream sources at a specific SHA. - `~/.cache/skill-engine/web-doc/<source_id>-<crawl_id>/` — snapshots of crawled web documentation for a specific crawl. Old flat-layout entries (`~/.cache/skill-engine/<source_id>-<sha>/`) may still exist if they predate the kind-partitioned layout and have not yet been migrated by REFRESH; this command will clean them too. <!-- doctrine:legacy-cache-layout --> The cache is regenerable: a subsequent DISCOVER or REFRESH against the same upstream re-creates whatever it needs from `gh`/`git` or the web crawler. Nothing outside the cache root is touched. No contextualizer state (`.claude/skills/<slug>-context/`, inc