parity(#243): add `csp clear orphans` to remove cached indexes whose source path no longer exists

[amondnet]

Summary

Upstream semble #243 (ca197f7) added semble clear orphans: removes cached indexes whose local source directory no longer exists. csp's clear only accepts all | index | savings.

Upstream

• cli.py::_clear_orphans(cache_folder): for each */index* entry, read metadata.json, take root_path; skip unless it is a non-empty string and cache_key(root_path) equals the entry's directory name (this excludes git-URL entries, whose root_path is a temp clone dir); delete when Path(root_path) does not exist.
• cache.py::cache_key(path) extracted so the CLI can verify the key.
• clear all does not include orphans; orphans is its own choice.

csp status

• crates/csp/src/bin/csp/main.rs::run_clear_at: choices ["all","index","savings"]. Not reflected.
• IndexManifest.source_id: Option<String> (indexing/index.rs) records the normalized source, and indexing/cache.rs::resolve_cache_dir derives the key from sourceId + content + ref — so an orphan check can recompute the key from the manifest and compare it to the directory name, as upstream does.

Tasks

• Add orphans to clear choices + CLEAR_CHOICES help text.
• clear_orphan_indexes(loc) in indexing/cache.rs: iterate <home>/index/*/manifest.json, skip git sources / key mismatches / unparsable manifests, remove entries whose source_id path no longer exists; keep the existing "only under the real index root" safety check.
• README.md + README.ko.md csp clear section.
• Tests against a temp cache dir (orphan removed, live entry kept, git entry kept, malformed manifest skipped).

Refs

• Upstream: feat: Add clear orphans command MinishLab/semble#243 (ca197f7)
• Sync baseline check: 949559b..6bbbd32, 2026-09-03